The effect on renal hemodynamics of Russell's viper (Vipera russelli siamensis) venom was studied in 8 mongrel dogs. The venom (0.10 mg/kg) was injected i.v. Measurements of general circulation and renal function were carried out over 48 hr. During the initial post-injection period, mean arterial blood pressure, pulse pressure and heart rate decreased. Total peripheral vascular resistance and renal vascular resistance showed a tendency to increase. There was no change in cardiac output. Thereafter the blood pressure and heart rate returned to the control level 2 hr after injection and remained stable throughout the experiment. The cardiac output remained unchanged, but the pulse pressure later increased. Renal blood flow, glomerular filtration rate, renal fraction and the rate of urine flow were decreased 24 hr after venom injection and rose to the control levels at 48 hr. Renal vascular resistance remained relatively increased, while peripheral resistance decreased, at 24 hr. Blood volume was unchanged throughout the 48 hr. Disseminated intravascular coagulation was not observed, although the clotting time was prolonged. Renal histological studies showed no remarkable changes; tubular necrosis was not seen. The renal hemodynamic changes may initially be due to catecholamine release and later to renin - angiotensin activation with renal vasoconstriction.