Congenital urinary tract obstruction is one of the most frequent malformations in fetuses or neonates, which usually causes profound impairment of renal function including reductions in both glomerular filtration rate (GFR) and tubular handling of water and solutes. Although obstruction can be released by surgical operation, the child will suffer from diuresis for sometime. It has been reported that erythropoietin (EPO) could prevent the down-regulation of aquaporin-2 (AQP2) and urinary-concentrating defects induced by renal ischemia/reperfusion (I/R) injury. However, whether EPO could promote the recovery of renal function and AQP2 expression after releasing of ureteral obstruction has not been reported yet. The purposes of the present study were to investigate the effects of EPO on renal function and AQP2 expression after release of bilateral ureteral obstruction (BUO-R) in rats. The results showed that EPO could promote interleukin (IL) 10 (IL-10) expression; inhibit tumor necrosis factor-α (TNF-α), IL-6, and inducible nitric oxide synthase (iNOS) expressions; reduce the fractional excretion of sodium (FENa) and plasma creatinine (CREA) and urea; and promote the recovery of water and salt handling and AQP2 expression in BUO-R rats, especially in the high dose of EPO-treated group rats. In conclusion, EPO could promote the recovery of renal function and AQP2 expression in BUO-R rats, which may partially associate with its anti-inflammation effect.

Vasoprotective effects of erythropoietin in animal models are mediated by endothelium-derived NO and/or mobilization of EPCs (endothelial progenitor cells) and may be enhanced by ischaemia: whether they are present in humans is unknown. We examined whether the erythropoietin analogue darbepoetin improves FMD (flow-mediated dilatation), a measure of endothelium-derived NO, and whether this is influenced by preceding I/R (ischaemia/reperfusion). A total of 36 patients (50–75 years) with stable coronary artery disease were randomized to receive a single dose of darbepoetin (300 μg) or saline placebo. FMD was measured at the brachial artery using high-resolution ultrasound. CD133 + /CD34 + /VEGFR2 + (vascular endothelial growth factor receptor 2) circulating EPCs were enumerated by flow cytometry. Measurements were made immediately before darbepoetin/placebo and at 24 h, 72 h and 7 days. At 24 h, FMD was repeated after 20 min of I/R of the upper limb. A further group of 11 patients was studied according to the same protocol, all receiving darbepoetin, with omission of forearm I/R at 24 h. Immunoreactive erythropoietin peaked at 24 h and remained elevated at approximately 50-fold of baseline at 72 h. FMD did not differ significantly between groups at 24 h (before I/R). At 72 h (48 h after I/R), FMD was greater (by 2.3±0.5% in the darbepoetin compared with the placebo group, a 66% increase over baseline; P <0.001) and greater than FMD at the same time point without preceding I/R ( P <0.01). Increases in CD133 + /CD34 + /VEGFR2 + cells after darbepoetin did not differ according to the presence or absence of preceding I/R. Preceding I/R is required for darbepoetin to enhance endothelial function, possibly by increasing expression of the erythropoietin receptor and by a mechanism likely to involve Akt/NO rather than circulating EPCs.

Transgenic overexpression of erythropoietin (Epo) in mice increases haematocrit to a mean of 80% in adult mice, leading to an increase in blood viscosity and volume. As a consequence, renal tissue endothelin-1 (ET-1) concentrations are significantly elevated in erythropoietin-overexpressing (Epo+) mice (mean±S.E.M; Epo+, 798±71; Epo-, 400±25pg/g tissue; P <0.01). To investigate the pattern of expression of the primary translation product of the ET-1 gene, prepro-ET-1, in kidneys of (Epo+) mice, we generated crossbred mice overexpressing the human EPO gene with mice carrying a reporter gene construct expressing the LacZ gene under the control of the human prepro-ET-1 promotor sequence (LacZ+/Epo+). For comparison, we generated (LacZ+/Epo-) mice from the same strains. After Bluo-Gal staining of frozen kidney sections ( n = 10 in each group), intracellular blue precipitates as indicators of prepro-ET-1 promotor activity were detectable in tubular and vascular endothelium and glomerular cells in (LacZ+/Epo-) as well as (LacZ+/Epo+) mice. Comparison of the amount of blue precipitates by semiquantitative scoring showed a significant increase in reporter gene activity in tubular epithelium of (LacZ+/Epo+) mice (mean±S.E.M.; LacZ+/Epo+, 1.64±0.18; LacZ+/Epo-, 1.00±0.19; P <0.05). Reporter gene activity was not significantly elevated in epithelium of small intrarenal arteries of (LacZ+/Epo+) mice (mean±S.E.M.; LacZ+/Epo+, 0.86±0.14; LacZ+/Epo-, 0.38±0.21; P = 0.08) and was similar in glomerular cells (mean±S.E.M.; LacZ+/Epo+, 1.28±0.16; LacZ+/Epo-, 1.14±0.21; P = 0.6). The main source of elevated ET-1 tissue concentration in kidneys of (Epo+) mice therefore seems more likely to be tubular than vascular endothelium or glomerular cells.

It is now widely known that erythropoietin (Epo) does not only affect the haematopoietic system, but it can be considered a multifunctional trophic factor with an effect on the general homoeostasis of the entire organism. The recent discovery of a specific Epo/Epo-receptor system in the central nervous system (CNS) and cerebrospinal fluid, independently of the haematopoietic system, has further paved the way for new studies aimed at investigating the different sites of cerebral expression of Epo and its receptor, the regulation of their expression and, finally, the effects that this hormone has on the development and maturation of the brain. A further aim has been to investigate how it influences CNS homoeostasis and neurotransmission in adult brain. Attention has also been focused on the neurotrophic and neuroprotective function of Epo in different conditions of neuronal damage, such as hypoxia, cerebral ischaemia and subarachnoid haemorrhage, and therefore on the possibility that human recombinant Epo therapy could soon be used in clinical practice, also to limit neuronal damage induced by these diseases.

This study investigated potential reasons why erythropoietin (EPO) given therapeutically to patients with renal failure may increase peripheral, but not renal, vascular resistance. This was done by comparing the effects of EPO on resting tension in normal renal interlobular and subcutaneous vessels from uraemic patients. In human subcutaneous arteries from uraemic subjects, noradrenaline- and KCl-induced vasoconstrictions were enhanced when nitric oxide (NO) production was blocked with N G -nitro- L -arginine methyl ester ( L -NAME), but were unaffected by EPO, while acetylcholine- and bradykinin-induced concentration-dependent relaxations were markedly attenuated by L -NAME, but not by EPO. The noradrenaline- and KCl-induced vasoconstrictions of human renal interlobular arteries were unaffected by the presence of L -NAME, but were attenuated by EPO (20 units·ml -1 ) by some 33% ( P < 0.01); this effect was enhanced by the co-administration of L -NAME. Acetylcholine and bradykinin caused comparable dilatations of the interlobular arteries; the response to the former was attenuated by L -NAME, but none of these responses were changed by EPO. EPO given alone, at a concentration of either 0.1 or 20 units·ml -1 , had no effect on basal resting tone. NO production mediated both acetylcholine- and bradykinin-induced relaxation in this vessel type. In contrast, in the interlobular arteries there was no indication of NO modulating the level of vasoconstriction, and it only mediated acetylcholine-induced dilation. These acute responses to EPO only partially explain its differential effects on the vasculature in renal failure.

We have investigated the effects of recombinant human erythropoietin (EPO) on the responses of rat renal arcuate arteries to dopamine, noradrenaline and acetylcholine and on the release of NO from human umbilical vein endothelial cells (HUVEC) in culture. Noradrenaline induced a concentration-dependent constriction and acetylcholine a concentration-dependent relaxation of the vessels. The effects of dopamine were concentration-dependent, leading to relaxation of the vessels at low concentrations and contraction of the vessels at high concentrations. N G -Nitro- L -arginine methyl ester ( L -NAME; 0.1 mM) did not change the vasoconstrictor responses to noradrenaline and dopamine, but inhibited the acetylcholine- and dopamine-induced vasorelaxation. Neither 0.1 nor 20 units·ml -1 EPO affected noradrenaline-induced constriction, or dopamine- or acetylcholine-induced relaxation, of the vessels. EPO at 20 units·ml -1 attenuated dopamine-induced constriction of the vessels. This effect was blunted by application of L -NAME, suggesting that EPO may stimulate dopamine-mediated NO release from these vessels. EPO stimulated NO release from the resting HUVEC in a concentration- and time-dependent manner, an effect that was inhibited by the presence of N G -nitro- L -arginine. These data suggest that, in vitro , EPO is able to stimulate NO release from rat renal arcuate arteries and HUVEC in culture. Whether these acute short-term actions can be related to the longer-term actions of EPO remains to be resolved.

1. Serum levels of erythropoietin and the immune parameters tumour necrosis factor-α, soluble interleukin-2 receptor, interleukin-2, interleukin-6 and interferon-γ were measured in patients with rheumatoid arthritis. 2. Out of 69 patients, 44 had anaemia with serum haemoglobin concentrations of 10.8 (SD 1.2) g/dl. In these patients erythropoietin levels were significantly higher than in non-anaemic patients [51.97 (SD 23.9) versus 26.06 (SD 11.9) m-units/ml; P < 0.0001; control patients: 18.1 (SD 13.8) m-units/ml]. Mean soluble interleukin-2 receptor activity was elevated in all patients with rheumatoid arthritis [1324 (SD 715) units/ml; control patients: 480 (SD 75) units/ml; P < 0.001] and was significantly higher in the anaemic group than in the non-anaemic group [1562 (SD 662) versus 696 (SD 402) units/ml; P < 0.0001]. The serum activity of soluble interleukin-2 receptor showed an inverse correlation with haemoglobin ( r = 0.79; P < 0.0001) and a positive correlation with erythropoietin ( r = 0.70, P < 0.0001). 3. Elevated serum tumour necrosis factor-α levels were found in 19 anaemic patients [20.6 (SD 9.1) pg/ml]. Concentrations of tumour necrosis factor-α in serum showed an inverse correlation with haemoglobin ( r = 0.57, P < 0.001) and a positive correlation with erythropoietin ( r = 0.46, P < 0.05). Interleukin-6 was detected in seven anaemic patients [21 (SD 14) pg/ml] and interleukin-2 activity in three anaemic patients (12, 16 and 14 units/ml, respectively). Interferon-γ was not detected in any of the patients investigated. 4. The present study supports the concept that in patients with rheumatoid arthritis the responsiveness to erythropoietin is impaired. Immune mechanisms may be operative and point to a pivotal role for tumour necrosis factor-α.

1. The effects of correcting anaemia on exercise capacity were evaluated in 21 haemodialysis patients (aged 39 ± 12 years) before starting treatment with recombinant human erythropoietin (Hb concentration, 73 ± 10 g/l; total Hb, 59 ± 12% of expected), after correction of the anaemia to a Hb concentration of 108 ± 7 g/l and a total Hb 82 ± 10% of expected, and in 13 of the patients after 12 months on maintenance recombinant human erythropoietin treatment (Hb concentration 104 ± 14 g/l, total Hb 79 ± 17% of expected). Fifteen healthy subjects (aged 41 ± 9 years), who took no regular exercise, constituted the control group. Maximal exercise capacity was determined on a bicycle ergometer. Oxygen uptake, respiratory quotient, blood lactate concentration, heart rate and blood pressure were measured at rest and at maximal workload. 2. After 6 ± 3 months on recombinant human erythropoietin, maximal exercise capacity increased from 108 ± 27 W to 130 ± 36 W ( P < 0.001) and the maximal oxygen uptake increased from 1.24 ± 0.39 litres/min to 1.50 ± 0.45 litres/min ( P < 0.001). No significant changes in respiratory quotient (1.16 ± 0.13 versus 1.18 ± 0.13) and blood lactate concentration (4.0 ± 1.8 versus 3.6 ± 1.1 mmol/l) at maximal workload were observed, but the blood lactate concentration in the patients was significantly lower than that in the control subjects (6.7 ± 2.3 mmol/l, P < 0.01). After the correction of anaemia, the aerobic power was still 38% lower in the patients than in the control subjects and 17% lower than the reference values. 3. After 12 months on maintenance recombinant human erythropoietin treatment (17 ± 3 months from the start of the study), no further significant changes were observed in maximal exercise capacity (before start, 112 ± 31 W, 6 ± 3 months, 134 ± 42 W, 17 ± 3 months, 134 ± 50 W), maximal oxygen uptake (before start, 1.33 ± 0.45 litres/min; 6 ± 3 months, 1.59 ± 0.54 litres/min; 17 ± 3 months, 1.75 ± 0.78 litres/min) or blood lactate concentration (before start, 4.4 ± 1.9 mmol/l; 6 ± 3 months, 4.0 ± 1.0 mmol/l; 17 ± 3 months, 4.7 ± 2.0 mmol/l). 4. Thus, in haemodialysis patients the improvement in maximal aerobic power after the correction of anaemia persists without marked changes during long-term treatment with recombinant human erythropoietin. We did not observe any effects on exercise capacity that could be attributed to a spontaneous increase in physical activity after treatment of anaemia.

1. To clarify the relationship between nocturnal oxygen desaturation and erythropoietin production in patients with chronic obstructive pulmonary disease, we determined arterial oxygen saturation and serum immunoreactive erythropoietin levels over 24 h in eight patients with chronic obstructive pulmonary disease and in nine normal subjects. 2. In the normal subjects, there was a significant circadian variation in serum erythropoietin levels with the highest mean deviation from the geometric mean at 22.00 hours and the nadir at 05.00 hours. 3. The three patients with chronic obstructive pulmonary disease with the most marked nocturnal desaturation (lowest arterial oxygen saturation <57%) and most marked daytime hypoxaemia (daytime arterial partial pressure of oxygen < 6 kPa) had raised nocturnal serum erythropoietin levels. In two of these patients, the serum erythropoietin level was raised throughout the 24 h and erythrocyte mass was also raised. In the other patient, the serum erythropoietin level was not raised in five daytime samples and erythrocyte mass was normal. 4. The other five patients with chronic obstructive pulmonary disease with less severe nocturnal hypoxaemia (lowest arterial oxygen saturation range 78–86%) had serum erythropoietin levels (range 14–36 m-i.u./ml) which were indistinguishable from normal (range 12–44 m-i.u./ml) and showed circadian changes which were not significantly different ( P = 0.35) from those in the normal subjects. 5. Thus, mild nocturnal oxygen desaturation is not associated with elevation of serum erythropoietin levels, whereas daytime hypoxaemia with associated severe nocturnal desaturation is associated with increased serum erythropoietin levels both by day and by night.

1. Elevation of blood pressure is one of the major side effects of recombinant human erythropoietin therapy in haemodialysis patients. 2. We investigated the possible involvement of endothelin in the pathogenesis of this recombinant human erythropoietin-induced blood pressure elevation in 51 patients undergoing maintenance haemodialysis. 3. Blood haemoglobin level increased from 7.1 ± 0.1 to 8.8 ± 0.1g/dl (means ± SEM) after 8 weeks of treatment with recombinant human erythropoietin (3000–4500 units/week). An increase in mean blood pressure was found in 19 patients (37%) ( n = 9, by 0–10 mmHg; n = 10, by > 10 mmHg). 4. Plasma immunoreactive-endothelin concentration significantly increased from 2.26 ± 0.18 to 3.14 ± 0.31 pmol/l in the 10 patients whose mean blood pressure increased by more than 10 mmHg ( P < 0.05), but not in the other patients. Moreover, the increase in plasma immunoreactive-endothelin concentration showed a significant positive correlation with the change in mean blood pressure in 19 patients with elevated mean blood pressure ( r = 0.47, P < 0.05). 5. There was no significant correlation between the change in plasma immunoreactive-endothelin concentration and the change in blood haemoglobin level or the change in body weight. 6. These results suggest the possibility that endothelin may contribute to the recombinant human erythropoietin-related rise in blood pressure in some haemodialysis patients.

1. Studies were undertaken to examine the effect of acute and chronic administration of human recombinant erythropoietin on kidney cortical and papillary perfusion in the anaesthetized rat using laser-Doppler flowmetry. 2. Thirty minutes after erythropoietin (50 and 150 units/kg intravenously), blood pressure, cortical perfusion and papillary perfusion were unchanged. 3. In animals treated chronically with erythropoietin over 7 days (three doses of 150 units/kg subcutaneously) blood pressure was similar to that of vehicle-treated animals, whereas cortical perfusion and papillary perfusion were reduced by 23% and 20%, respectively (both P <0.05), and the packed cell volume (51.1 ± 0.7%) was significantly ( P <0.01) greater than in vehicle-treated animals (46.2 ± 0.6%). 4. Bolus doses of vasopressin and phenylephrine increased blood pressure (by between 10% and 40%) and decreased cortical and papillary perfusion (by between 10% and 20%), while angiotensin II caused similar increases in blood pressure and decreases in cortical perfusion but not papillary perfusion. The magnitude and pattern of these responses were comparable after both acute and chronic administration of erythropoietin. 5. Erythropoietin given acutely at therapeutic levels has a marginal effect on cortical and papillary perfusion. However, the chronic treatment indicated that there was a sustained reduction in both cortical and papillary perfusion, reflecting a vasoconstriction. This reduction in renal haemodynamics could contribute to the increase in blood pressure observed when this hormone is administered in man.

1. In experimental studies, activation of renal sympathetic nerves attenuates the natriuretic response to atrial natriuretic factor. We therefore investigated the response to low-dose infusion of atrial natriuretic factor in renal transplant recipients. 2. Eight male cyclosporin-treated renal transplant recipients received human-α atrial natriuretic factor (1–28) at a dose of 1.2 pmol min −1 kg −1 or placebo for 2 h in a placebo-controlled, randomized, cross-over study. The plasma atrial natriuretic factor concentration rose from 18.5 to 49.2 pmol/l in association with an immediate natriuresis (a rise of 49.1 μmol/min in the first 30 min, P <0.05; peaking at a 61% increase from baseline, P <0.01), diuresis (from 3.37 to 7.46 ml/min) and a threefold rise in urinary cyclic GMP excretion. 3. In response to infusion of atrial natriuretic factor, the packed cell volume rose by 4.2% ( P < 0.001) and the filtration fraction by 5% (from 22 to 27%, P <0.05), but there was no significant change in renal plasma flow, glomerular filtration rate or mean arterial blood pressure. Likewise, the plasma catecholamine concentrations, plasma renin activity and serum erythropoietin concentration remained unchanged. 4. The sensitivity of the renal allograft to plasma atrial natriuretic factor concentrations in the high physiological range suggests a role for endogenous atrial natriuretic factor in the modulation of graft function. Furthermore, the immediate natriuretic response to atrial natriuretic factor in the effectively denervated transplant kidney, in contrast to the delayed response seen in normal subjects, may imply that sympathetic nerves have an inhibitory effect on the renal response to atrial natriuretic factor in normal man. 5. In contrast to the findings of studies in vitro , infusion of atrial natriuretic factor had no effect on the serum erythropoietin concentration.

1. The ATP production rate in isolated skeletal muscle mitochondria was measured with a bioluminescence method, before and during erythropoietin treatment, in 21 anaemic haemodialysis patients. In addition, the concentrations of ATP, phosphocreatine and total creatine and the ratio of alkali-soluble protein to DNA were determined in skeletal muscle. Maximal oxygen uptake and maximal exercise capacity were determined on a bicycle ergometer. 2. The results unexpectedly showed a 35% higher mitochondrial ATP production rate in the patients before erythropoietin treatment than in sedentary control subjects. On the other hand, mitochondrial density, as measured by the activity of the matrix enzyme glutamate dehydrogenase, was the same in the patients as in the sedentary control group. After 1 year on maintenance erythropoietin treatment, the ATP production rate per kg of muscle decreased in five out of seven patients and reached the same level as in the sedentary control subjects. The ratio between ATP production rate and glutamate dehydrogenase activity was on average 40% higher in the patients at the start and decreased towards the control level in six out of seven patients after 1 year on maintenance erythropoietin treatment. When related to the mitochondrial protein content, a significant reduction in the ATP production rate was observed. 3. The ratio of alkali-soluble protein to DNA in skeletal muscle and the concentrations of ATP, phosphocreatine and total creatine in skeletal muscle at rest were normal in the patients and did not change during the study. The maximal aerobic power improved by 25% after the correction of anaemia. 4. These results suggest that the enhanced mitochondrial ATP production rate in renal anaemia is a metabolic adaptation to decreased oxygen transport and is reversed by long-term correction of anaemia.

1. A randomized, partial-crossover study was conducted in uraemic patients with dialysis-associated anaemia and transfusional iron overload to evaluate the effects of desferrioxamine chelation therapy and of recombinant human erythropoietin treatment on hepatic iron storage determined by computed tomography, as well as by serum ferritin concentration and transferrin saturation. 2. Twenty-one haemodialysis patients with moderate iron overload, confirmed by values of serum ferritin concentration, transferrin saturation and hepatic computed tomography density exceeding 1000 μg/l, 45% and 68 Hounsfield units respectively, were randomly allocated to three groups and were followed for 12 months. 3. During the first 6 months group 1 ( n = 7) received desferrioxamine chelation therapy (30 mg/kg intravenously three times a week) and group 2 ( n = 7) underwent recombinant human erythropoietin treatment (36 units/kg intravenously three times a week). Thereafter, in the second 6 months of observation patients in group 1 were switched to receive recombinant human erythropoietin. Because of a poor response in the desferrioxaminetreated group in the initial 6 months, patients in group 2 continued on the maintenance dose of recombinant human erythropoietin (18 units/kg three times a week) until the end of the trial. Patients in group 3 ( n = 7) were maintained on placebo throughout the study. 4. In comparison with placebo, recombinant human erythropoietin treatment, but not desferrioxamine chelation therapy, reduced serum ferritin concentration, transferrin saturation and hepatic computed tomography density, and was associated with a rise in haemoglobin and packed cell volume. Hepatic computed tomography density, serum ferritin concentration and transferrin saturation decreased in 13 out of 14 patients (93%) during treatment with recombinant human erythropoietin. However, when the changes in hepatic computed tomography density were compared with those in the biochemical indices, we observed that the decreases in serum ferritin concentration and transferrin saturation were much slower and delayed. More specifically, within 6 months of starting recombinant human erythropoietin treatment, hepatic computed tomography density was normalized in 13 out of 14 patients (93%), whereas serum ferritin concentration and transferrin saturation were within the normal limits in only two (14%) and six patients (43%), respectively. 5. In conclusion, the strategies for monitoring the iron status of haemodialysis patients with transfusional haemosiderosis may evolve to a new level of sophistication with the introduction of computed tomography scanning. This technique has the advantage of estimating directly the effect of recombinant human erythropoietin treatment on hepatic iron storage. Hepatic computed tomography density is complementary to serum ferritin concentration and transferrin saturation in monitoring the iron status of haemodialysis patients treated with recombinant human erythropoietin.

1. Polycythaemia occurs in man secondary to chronic hypoxaemia, and may lead to morbidity from hyperviscosity of the blood. Hypoxaemic rats develop similar changes. We have investigated the effect of the calcium antagonist verapamil upon the polycythaemic response to hypoxia in rats. 2. Control groups of 10 male rats breathed air in an environmental chamber for 28 days. Hypoxic groups breathed a normobaric atmosphere of 10% oxygen for 6 h each day, and air for the remaining 18 h. Control and hypoxic groups were treated with intraperitoneal or subcutaneous water, or with intraperitoneal or subcutaneous water plus verapamil. 3. On day 28, packed cell volume (PCV) was measured by a microhaematocrit technique and red cell mass (RCM) by dilution of injected 51 Cr-labelled rat erythrocytes. 4. PCV and RCM were significantly higher in all the hypoxic groups compared with the control groups ( P < 0.01 in each case). 5. PCV and RCM were significantly lower in the hypoxic groups treated with intraperitoneal or subcutaneous verapamil than in the hypoxic groups treated with intraperitoneal or subcutaneous water ( P < 0.01). There were no significant differences between PCV and RCM in verapamil- and water-treated normoxic control groups. Verapamil had no effect on the shift of the oxygen/haemoglobin dissociation curve produced by hypoxia. 6. Verapamil reduces the polycythaemic response to repeated intermittent hypoxia in rats. Venesection is usually performed for excessive secondary polycythaemia in man. Our results suggest a possible therapeutic role for verapamil in such individuals.

1. We studied 20 patients with chronic airflow obstruction, 10 patients without polycythaemia and 10 patients with compensatory polycythaemia having respectively mean red cell mass 24.7 (sd 4.2) and 47.8 ( sd 7.5) ml/kg, mean daytime P ao 2 7.6 and 6.9 kPa, mean FEV, 0.85 and 0.82 1. Groups were matched for severity of daytime arterial hypoxaemia but nocturnal arterial oxygen desaturation was more severe in the patients with polycythaemia than in those without. We also studied six additional patients with chronic airflow obstruction and polycythaemia and 19 normal controls. 2. Estimates of serum immunoreactive erythropoietin (siEp) in those without polycythaemia were 19 m-i.u./ml (geometric mean) with 95% confidence range 11–35 m-i.u./ml and stable during 3 months. In those with polycythaemia they were similar and consistent in five and, in the other five, higher on at least one occasion. There was no significant difference between siEp in daytime (12.00 hours to 16.00 hours) and morning (07.00 hours) samples but geometric mean estimates of erythropoietin in paired daytime and morning samples were higher and more variable in patients with polycythaemia than in those without. The geometric mean estimate of siEp in all patients with chronic airflow obstruction and polycythaemia was greater than in normal subjects but, despite secondary polycythaemia, siEp could be in the range for normal subjects. 3. In the patients with polycythaemia we were unable to predict the finding of normal or elevated siEp. 4. Changes in siEp after erythrapheresis (10–26% reduction in packed cell volume) were observed in the 10 patients with polycythaemia and in one without. One month after erythrapheresis, packed cell volume remained below and siEp was above initial pretreatment levels, implying an erythropoietin secretory response and that the development of secondary polycythaemia had induced a fall in siEp.