The influence of gradual decline in renal perfusion pressure (RPP) due either to renal artery clamping (C) or to haemorrhagic hypotension (HH) was studied using micropuncture techniques in anaesthetized dogs. The decrease in renal blood flow (RBF) was more profound and set in earlier during HH than during C, where perfect autoregulation was observed down to a mean arterial blood pressure of 85 mmHg. Glomerular filtration rate (GFR) was also only slightly decreased during C, with no change in filtration fraction (FF); again, a much greater decrease in GFR with an increase in FF was seen in HH. The excretion of water, electrolytes and urea were also more decreased during HH than during C. Similar changes were seen at the single nephron (SN) level. Opposite changes were observed in arteriolar resistances: during C a decrease in total arteriolar resistance (RT) amounting to -22% at a RPP of 84 mmHg and -13% at 60 mmHg was seen, due exclusively to a drop in afferent resistance (RA), but during HH there was a significant increase in RT by +36% at RPP of 110 mmHg, +39% at 85 mmHg and +68% at 60 mmHg. This increase was mainly due to an increase in efferent resistance (RE) rather than in RA: +42 vs. +31%, respectively, at 110 mmHg and +67 vs +19% respectively, at 85 mmHg. It was not until a RPP of 60 mmHg was reached that this difference between RE and RA disappeared, being +67% for RE and +69% for RA. The ultrafiltration coefficient, Kf, did not change during C and only decreased slightly with the biggest drop in RPP during HH (2.84 microliters mmHg-1 min-1 during HH vs. 4.19 microliters mmHg-1 min-1 before HH). The SNGFR/GFR ratio remained unchanged during C but declined with decreasing RPP during HH, which probably indicates a 'redistribution' of RBF to the deeper regions of the renal cortex. In conclusion, major differences in renal function were observed between C and HH whose cause is unknown.