1. The response of arterial blood pressure, plasma renin activity and plasma aldosterone concentration to inhibition of angiotensin I converting enzyme (kininase II) with captopril has been studied in patients with severe, treatment-resistant, malignant hypertension.
2. Nine patients with a past history of severe hypertension, supine diastolic blood pressure > 120 mmHg before conventional antihypertensive therapy and resistant to conventional antihypertensive therapy were studied.
3. Captopril administration resulted in a marked decrease in arterial blood pressure and plasma aldosterone concentration and an increase in plasma renin activity.
4. Although arterial blood pressure remained significantly below the values observed during the control period, pressure did tend to increase again after 3 days. Addition of hydrochlorothiazide kept arterial pressure significantly below pretreatment control values.