SUMMARY Conscious nonnotensive and two-kidney, one clip Goldblatt hypertensive dogs were studied to determine the influence of the sympathetic and renin-angiotensin systems on the blood pressure and renal blood flow. Renal blood flow was monitored in a single kidney of the normotensire dogs and in the contralateral kidney of the hypertensive dogs. Saralasin was infused intravenously (i.v.) at 1 $g/kg/min for 15 minutes in untreated and guanethidine-treated animals. Guanethidine (i.v.) decreased blood pressure (BP) in the hypertensives at X = 16 days after Goldblatt clamp application, but not in the normotenslves, and did not affect renal blood flow or vascular resistance in either group. Saralasin produced approximately the same BP decrease in both untreated and guanetbidine-treated hypertensives at X = 12 days after Goldblatt clamp application; the combined effect of the drugs was greater than saralasin alone. Blood flow and vascular resistance of the hypertensives' contralateral kidney were not significantly affected by saralasin in these experiments. No sustained effect on BP or renal blood flow was obtained with saralasin in the normotensires. Administration of furosemide was utilized to increase plasma renin activity (PRA) in another series of experiments. In both normotensives and hypertensives, PRA was elevated at 2-3 hours after furosemide application, being greater in the hypertensives. When the renin-angiotensin system was activated in this manner, saralasin had a significant hypotensive and renal vasodilator effect in the hypertensives, but not in the normotenslves.