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.

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