The efficacy and safety of prazosin, a new vasodilator antihypertensive agent, used mainly in combination with other hypotensive medication, was evaluated in 50 hypertensive patients, 28 of whom had left ventricular hypertrophy and 28 of whom had renal impairment. In 44 cases this agent was introduced because of an inadequate response to, or side effects from other agents. In 38 cases (76%) there was a satisfactory response, the diastolic blood pressure being reduced to 100 mm Hg or less. The blood pressure of the whole group of 50 patients was reduced by an average of 31/20 mm Hg. The mean duration of therapy was 6-6 months. Side effects necessitating withdrawal of prazosin occurred in only three cases. Plasma renin activity was reduced to 25% of the control value in a group treated with prazosin alone.