A parenteral preparation of captopril has been used to produce a smooth reduction of blood pressure in patients presenting with accelerated phase hypertension. In five out of six patients studied, an intravenous infusion of captopril at dose rates ranging from 250-2000 micrograms/h lowered blood pressure from 199 +/- 13/115 +/- 5 to 143 +/- 14/86 +/- 5 mmHg over several hours without adverse effects. Partial inhibition of the angiotensin converting enzyme was demonstrated by a rise in plasma renin activity (PRA) and a fall (although not to normal) in elevated levels of angiotensin II and aldosterone. The plasma level of free captopril at the point of blood pressure control was 15 +/- 4 ng/ml and its short effective half-life was demonstrated by a rise in blood pressure within 15 minutes of stopping the infusion. These data demonstrate that very small amounts of captopril can produce a dose-dependent inhibition of angiotensin converting enzyme without abrupt changes in blood pressure.