1. Erythrocyte choline transport has been studied in nine patients on maintenance haemodialysis for chronic renal failure, six patients on continuous ambulatory peritoneal dialysis, 31 patients with renal transplants and in nine normal control subjects. 2. The mean maximum rate of choline influx ( V max. , measured at an extracellular choline concentration of 250 μmol/l) was 66.7 ( sd 14.1) μmol h −1 l −1 cells in patients on haemodialysis, 87.8 ( sd 18.5) μmol h −1 l −1 cells in patients on continuous ambulatory peritoneal dialysis and 30.5 ( sd 4.9) μmol h −1 l −1 cells in control subjects. The increase in choline flux in patients on haemodialysis and patients on continuous ambulatory peritoneal dialysis compared with control subjects was highly significant ( P < 0.001). 3. Renal transplant patients showed variable values for the V max. of choline influx (range 17.7-71.7 μmol h −1 l −1 cells). The values showed a signifcant negative correlation with creatinine clearance and this correlation correctly extrapolated to the maximum choline flux in normal subjects and in patients on dialysis. 4. The kinetics of choline transport have been studied in erythrocytes of patients on haemodialysis and control subjects in ‘zero-trans’ conditions after depletion of intracellular choline. The mean V max. in these conditions was 38.4 ( sd 4.6) μmol h −1 l −1 cells in patients on haemodialysis compared with 14.2 ( sd 3.7) μmol h −1 l −1 cells in control subjects. The mean K m under ‘zero-trans’ conditions was 19.4 ( sd 2.4) μmol/l in patients on haemodialysis and 7.4 ( sd 1.4) μmol/l in control subjects. These differences were significant ( P < 0.001).

1. The initial rate of l -lysine influx into erythrocytes from 13 patients with chronic renal failure has been measured using 14 C-labelled lysine. Ten patients were on maintenance haemodialysis and three had never been dialysed. The results are compared with data obtained from 12 normal individuals. 2. The rate of lysine influx into washed cells from buffered saline containing 0.02–0.5 mmol of l -lysine/l has been calculated. The results can be fitted with a model in which influx has a single saturable component obeying Michaelis–Menten kinetics, and a linear non-saturable component. 3. In uraemic erythrocytes the saturable component had a mean V max. of 0.762 mmol h −1 litre −1 of cells ( n = 13, sem 0.072) and a mean K m of 68.2 μmol/l ( sem 5.7). These values in normal erythrocytes were 0.566 mmol h −1 litre −1 of cells ( n = 12, sem 0.033) and 70.5 μmol/l ( sem 4.1), respectively. The mean apparent diffusion constant ( K D ) for the linear component of influx was 0.224 h −1 ( sem 0.039) in uraemic cells and 0.178 h −1 ( sem 0.028) in normals. 4. The 35% increase in mean V max. seen in uraemic erythrocytes was statistically significant ( P = 0.02). A similar increase in V max. in uraemic cells compared with controls was seen in erythrocytes which were studied in zero-trans conditions after depletion of intracellular amino acids. The mean values of K m and K D were not significantly different in uraemia. The origins of this increased membrane transport capacity for lysine in uraemia are discussed.