1. The effect on renal function in sheep of inhibiting converting enzyme with captopril was examined before and after 5 days ACTH administration. 2. Glomerular filtration rate, effective renal plasma flow, effective renal blood flow, mean arterial pressure and plasma sodium were all significantly increased by ACTH treatment and plasma potassium was decreased. Captopril (20 mg i.v.) had no effect on renal function or blood pressure before or after ACTH treatment, although urinary potassium excretion decreased following captopril on day 6 of ACTH treatment. 3. The increase in glomerular filtration rate and effective renal plasma flow seen with ACTH treatment in sheep does not appear to be mediated by the reninangiotensin system.