1. This study in conscious dogs examined the quantitative effects of a reduction in the renal arterial pressure on the renal homoeostatic responses to an acute extracellular fluid volume expansion. 2. Seven female beagle dogs were chronically instrumented with two aortic catheters, one central venous catheter and a suprarenal aortic cuff, and were kept under standardized conditions on a constant high dietary sodium intake (14.5 mmol of Na + day −1 kg −1 body weight). 3. After a 60 min control period, 0.9% (w/v) NaCl was infused at a rate of 1 ml min −1 kg −1 body weight for 60 min (infusion period). Two different protocols were applied during the infusion period: renal arterial pressure was maintained at 102 ± 1 mmHg by means of a servo-feedback control circuit (RAP-sc, 14 experiments) or was left free (RAP-f, 14 experiments). 4. During the infusion period, in the RAP-sc protocol as well as in the RAP-f protocol, the mean arterial pressure increased by 10 mmHg, the heart rate increased by 20 beats/min, the central venous pressure increased by 4 cmH 2 O and the glomerular filtration rate (control 5.1 ± 0.3 ml min −1 kg −1 body weight, mean ± sem ) increased by 1 ml min −1 kg −1 . 5. Plasma renin activity [control 0.85 ± 0.15 (RAP-f) and 1.08 ± 0.23 (RAP-sc) pmol of angiotensin I h −1 ml −1 ] decreased similarly in both protocols. 6. Renal sodium excretion, fractional sodium excretion and urine volume increased more in the RAP-f experiments than in the RAP-sc experiments ( P <0.05), renal sodium excretion from 8.2 to 70.1 (RAP-f) and from 7.7 to 47.4 (RAP-sc) μmol min −1 kg −1 body weight, fractional sodium excretion from 1.1 to 8.0 (RAP-f) and from 1.0 to 5.4 (RAP-sc)% and urine volume from 39 to 586 (RAP-f) and from 38 to 471 (RAP-sc) μl min −1 kg −1 body weight. 7. In the RAP-f experiments as well as in the RAP-sc experiments, urinary sodium excretion increased with expansion of the extracellular fluid volume, which increased by a maximum of 21% (fasting extracellular fluid volume: 206 ± 4 ml/kg body weight, six dogs, 28 days). 8. The increase in renal arterial pressure contributed significantly to the renal homoeostatic response, as 21% less urine and 31% less sodium were excreted when the extracellular fluid volume was expanded and the renal arterial pressure was kept constant below control pressure rather than being allowed to rise. The differences in sodium and water excretion were mainly due to the effect of renal arterial pressure on tubular reabsorption. However, the striking increase in sodium and urine excretion which occurred despite the reduction in renal arterial pressure emphasizes the importance of other homoeostatic factors involved in body fluid regulation.