The value of the orally active converting enzyme inhibitor captopril in managing hypertensive crisis was tested in 9 untreated patients admitted to the emergency room, who were in need of rapid blood pressure reduction because of signs and symptoms of neurological and/or cardiac complications. During the 30 min following administration of captopril 25 mg the blood pressure decreased from 239/134 +/- 12/4 mmHg (mean +/- SEM) to 204/118 +/- 8/4 mmHg (p less than 0.05). From that time on, captopril 200 to 300 mg/day was continued for 2 to 5 days. In 5 patients furosemide in a total dose of 40 to 160 mg i.v. or p.o. had also to be given in order to control the blood pressure. 12 and 24 h after admission blood pressure averaged 140/93 and 139/86 mmHg respectively, in the patients treated with captopril alone, and 166/107 and 153/91 mmHg in those treated both with captopril and furosemide. The pronounced fall in blood pressure produced by blockade of the renin system was well tolerated and did not cause tachycardia. It appears, therefore, that captopril given alone or in association with a diuretic makes it possible to treat the hypertensive crisis without the need for monitoring in an intensive care unit.