We have investigated the use of captopril as a screening test for renovascular hypertension and compared the effects of captopril on renal function in patients with renovascular hypertension and those without renovascular hypertension. The captopril test and kidney gamma scintigraphy were carried out in 50 hypertensive patients, 13 with renovascular hypertension and 37 without. Blood samples were drawn for the determination of plasma renin activity and kidney gamma scintigraphy was done before and 60 minutes after 50 mg oral captopril administration. Results suggesting the diagnosis of renovascular hypertension are the following: a basal and stimulated plasma renin activity of 4 ng ml/hr or more and an absolute increase in plasma renin activity of 3 ng/ml/hr or more if basal plasma renin activity was less than 4 ng/ml/hr. Data from kidney gamma scintigraphy showed that captopril causes a decrease in clearance rate at 20 minutes in patients with renovascular hypertension but not in patients without renovascular hypertension. We conclude that the captopril test can be used to screen for renovascular hypertension, but catopril may impair the renovascular hypertensive patient's renal function.