1. Arterial plasma renin activity was significantly elevated in rats with one-kidney, one-clip hypertension of less than 3 weeks duration.
2. Intraperitoneal injection of the angiotensin-converting enzyme inhibitor SQ 14 225 (captopril) caused a dose-related decrease in systolic blood pressure in hypertensive rats. The lowest dose of captopril used (3.5 mg/kg) inhibited conversion of exogenous angiotensin I and maximally potentiated the depressor response to bradykinin, but failed to restore blood pressure to that of the normotensive controls.
3. Removal of the solitary clipped kidney also did not restore blood pressure to normal. Injection of captopril (3.5 mg/kg) 24 h after nephrectomy, when no circulating renin activity was detectable, lowered blood pressure further in hypertensive but not in similarly nephrectomized controls.
4. These results indicate that raised blood pressure in early one-kidney, one-clip hypertension in the rat cannot be entirely attributed to the renin-angioterisin system, even when plasma renin activity is significantly increased.
5. This study has also confirmed a hypotensive action of captopril in anephric rats when plasma renin activity is undetectable.