Changes in plasma levels of active and inactive renin after the treatment with enalapril maleate (MK-421), a new angiotensin converting enzyme inhibitor, were studied in five patients with renovascular hypertension (RVH) due to unilateral renal artery stenosis. The dosage was increased when the blood pressure (BP) was not normalized for more than 3 days. Blood sampling was performed before, and 5 hours and 24 hours after the first administration, and on the 3rd day with each dosage. Active and inactive renin concentrations (ARC and IRC) showed a reciprocal change in 4 cases, 5 hours after the first dose. In the chronic treatment, ARC and IRC before the morning dose did not change apparently until the BP was normalized, when both ARC and IRC were evidently increased. It was suspected that a conversion from inactive to active renin may occur in the patients with RVH, when the active renin secretion is stimulated suddenly by the first dose of MK-421. The chronically diminished perfusion pressure in the kidney may stimulate the secretion of inactive renin, but the decrease in endogenous angiotensin II may not.