1. The effects of the new oral angiotensin-converting enzyme (ACE) inhibitor MK-421 on blood pressure, plasma renin activity, angiotensin-II, aldosterone and converting enzyme activity were assessed in 16 hypertensive patients followed for 6--18 weeks. 2. After an initial titration period, maintenance doses of 2.5--40 mg once daily produced satisfactory blood pressure control in 11 and a partial but significant decrease in the remainder. 3. Treatment was associated with no change in heart rate and no orthostatic hypotension. At 24 h after the first effective dose, blood pressure was still decreased, plasma ACE was suppressed to 16% of the baseline, plasma angiotensin to 58%, aldosterone to 42%, and plasma renin activity was elevated to 228% of the baseline. 4. Magnitude of blood pressure fall was significantly correlated with the height of pretreatment blood pressure but not with baseline levels of plasma renin activity.