Comparative effects of angiotensin-converting enzyme inhibitor (ACEI) captopril and calcium entry blocker nifedipine on the split renal function were studied in six patients with renovascular hypertension (RVH) with unilateral stenosis (38 +/- 5 years). Both captopril and nifedipine showed potent antihypertensive effects; the mean arterial pressure was reduced from 131 +/- 16 to 105 +/- 8 mm Hg (P less than 0.001) by captopril, and from 128 +/- 15 to 109 +/- 11 mm Hg (P less than 0.001) by nifedipine. The glomerular filtration rate (GFR) in the stenotic kidney was 24 +/- 6 mL/min, which decreased to 11 +/- 2 mL/min (P less than 0.01) during captopril administration, and only slightly decreased (18.8 +/- 5 mL/min) during nifedipine administration. The effective renal plasma flow (ERPF) increased in the nonstenotic kidneys in response to both drugs. These results show that angiotensin II rather than renal perfusion pressure may be important for maintaining GFR in the stenotic kidney, and further suggest that nifedipine may be used relatively safely in treating patients with RVH, although caution should be exercised in terms of their ability to preserve the renal function in individual kidneys.