Administration of captopril to animals with two-kidney, one clip, renovascular hypertension (RH) lowers the glomerular filtration rate (GFR) in the clipped kidney. The authors therefore tested the hypothesis that a decrease in GFR after captopril administration would identify patients with RH. Total GFR was measured by the plasma disappearance of Tc-99m-diethylenetriaminepentaacetic acid (DTPA) after bolus injection and single-kidney GFR from renal uptake of DTPA assessed by renography. The authors studied six patients with arteriosclerotic RH who had strongly lateralizing renal vein renin levels and greater than 80% stenosis of the renal artery to that kidney. Results were contrasted with those of six patients with essential hypertension (EH) with a similar mean arterial blood pressure (MABP). Captopril (50 mg orally) increased total GFR (ml/min) in all patients with EH (102 +/- 8 to 120 +/- 12, P less than 0.005). However, GFR decreased in patients with RH (73 +/- 8 to 61 +/- 9, P less than 0.05) after captopril. Although the single-kidney GFR of patients with RH decreased in all six stenotic kidneys (27 +/- 4 to 21 +/- 5, P less than 0.02), it did not change consistently in the contralateral kidneys (45 +/- 8 to 40 +/- 6, N.S.). Clonidine (0.3 mg) also lowered MABP in patients with RH but, unlike captopril, it did not reduce total kidney GFR (75 +/- 10 to 79 +/- 11, N.S.). In conclusion, short-term captopril administration increases GFR in patients with EH, but decreases it in those with RH. This action is unrelated to its depressor response.(ABSTRACT TRUNCATED AT 250 WORDS)