Two hundred seventy-four doses of diazoxide were rapidly administered intravenously to 101 severely hypertensive patients, 82 of whom had been unsuccessfully treated with a variety of other antihypertensive agents. Prompt hypotensive responses occurred after 215 doses administered to 83 patients; for those patients, the average diastolic blood pressure was 137 mm. Hg before the injections of diazoxide and 86 mm. Hg within five minutes after the injection. The duration of the hypotensive response to a single injection exceeded three hours for 80 per cent of the effective doses, and it was longer than six hours after 55 per cent of the doses. Although 66 of the 101 patients were azotemic at the time of their hypertensive crises, only 14 had appreciable increases in the concentration of blood urea after injections of diazoxide. Adverse effects were infrequent, transient and of little consequence in this series of patients. Nevertheless, diazoxide should not be used when sudden great reductions in blood pressure might be detrimental. © 1969.