Eight hypertensive patients with normal renal function and receiving diuretic therapy for at least 28 days received captopril in small increasing doses (6.25, 12.5, and 25 mg). Supine and standing blood pressure (BP) and pulse rate, plasma renin activity (PRA), and plasma aldosterone concentration (PAC) were measured before and after captopril administration. The mean (±SEM) maximal decline in BP was 38/18±3/2, 34/18±4/2, and 25/17±3/2 mm Hg and occurred within 70 minutes of each of the three doses. In contrast, the duration of a 10 mm Hg or greater decrease in BP was prolonged markedly (103±5, 175±15, and 287±10 minutes) after each dose increment. After captopril, mean PRA levels increased while PAC levels fell. Transient dizziness on standing occurred in two patients, but captopril was otherwise well tolerated. Evaluation of the response to initial doses of captopril appears to be helpful in predicting maintenance requirements.