Blood pressure (BP), plasma renin concentration (PRC), and 99mTc-labeled diethylenetriaminepenta acetate (DTPA) renography with determination of single kidney 99mTc-DTPA clearance and parenchymal mean transit time (MTT) were measured in exactly the same way on two consecutive days in 14 patients with renovascular hypertension (RVH), unilateral renal artery stenosis in nine and bilateral stenosis in five, and ten patients with essential hypertension (EH). The examination on day 1 served as a control for day 2 during which captopril (25 mg) was given orally one hour before measurements of PRC and DTPA clearance. Blood pressure was reduced by captopril in both groups, but the maximum decrease in systolic BP was slightly more pronounced (P less than .01) in RVH (22%, median) than EH (13%). Plasma renin concentration increased to a much greater extent (P less than .01) after captopril in RVH (366%) than in EH (46%), Single kidney 99mTc-DTPA clearance was significantly (P less than .01) reduced (-39.5%) and MTT considerably prolonged (170%) on the affected/most affected side in RVH, but both parameters were only slightly changed or unchanged on the unaffected/least affected side (-6.5%, -2% respectively) and were not significantly changed in any of the sides in EH. The degree of renal artery stenosis was significantly correlated to the increase in PRC (rho = -0.786, n = 14 patients, P less than .01), to the reduction in single kidney 99mTc-DTPA clearance (rho = 0.729, n = 19 kidneys, P less than .01) and to the prolongation in MTT (rho = -0.785, n = 16 kidneys, P less than .01). By analysis of the captopril-induced changes in 99mTc-DTPA clearance and MTT, it was possible to predict the existence of a moderate to several renal artery stenosis in arterial hypertension with a very high degree of probability, and the use of changes in 99mTc-DTPA clearance and MTT after angiotensin-converting enzyme (ACE) inhibition may become a valuable tool in differentiation between RVH and EH.