The effects of enalapril maleate (MK-421), a new angiotensin converting enzyme inhibitor, were studied on 5 patients with renovascular hypertension (RVH) due to unilateral renal artery stenosis. The therapeutic dosage was increased when the blood pressure (BP) was not controlled for more than 3 days. Blood sampling was performed before, and 5 hr and 24 hr after the first administration, and on the 3rd day with each dosage. The BP was normalized on 5 mg/day in 1 case, 10 mg in 1 case, 20 mg in 2 cases, and 40 mg plus mefruside in 1 case. Plasma renin activity (PRA) was significantly increased after 5 hr and recovered after 24 hr with 2.5 mg of the enalapril maleate, when the BP was not affected. This indicates that the increase in PRA is likely due to the reduced negative feedback of angiotensin II. When the blood pressure was lowered, PRA was increased and plasma aldosterone concentration (PAC) was decreased significantly. This rise of PRA may depend not only on the reduced negative feedback but also on the fall of BP. It is also considered that the PAC was decreased through the decrease in plasma angiotensin II. A fall of the glomerular filtration rate in one case and also a fall of the perfusion of the kidney of the stenotic side in another case were observed by radioisotope renograms. MK-421 administration was a useful treatment for RVH, and clearly normalized the BP of all the patients studied. However, there was a risk of a fall of renal function on the stenotic side due to the decrease in perfusion pressure.