The response to acute oral administration of 50 mg of Captopril was assessed in 17 normal volunteers and 47 patients with hypertension; 17 had renovascular (RVH) abnormalities and 30 patients had essential hypertension (EH). All patients were maintained on ad libitum sodium intake. The effect of Captopril on mean arterial pressure (MAP) was rapid a maximal within 60 minutes. The converting enzyme inhibitor induced a similar decrease in MAP in normal subjects (-5.1 +/- I mm Hg) and patients with EH (-7.2 +/- I mm Hg). Control plasma renin activity (PRA) was similar in both groups; however, the increase in PRA following Captopril was more marked in normals (8.1 +/- 1.7 ng/ml/h) than in EH (1.7 +/- 0.7 ng/ml/h). In patients with RVH a marked fall in MAP occurred (-25.4 +/- 4 mm Hg). A fall in MAP higher than 20 mm Hg was observed in 65% of patients with RVH and none of the EH group. A negative correlation between log PRA and the change in MAP induced by Captopril was obtained (r = 0.65). Assessment of the response to acute administration of Captopril may be useful for screening patients with RVH.