Fifteen patients with severe essential or renal hypertension were treated with captopril. In thirteen, a single oral dose of 1 mg/kg produced a mean fall of 20.7 +/- 16.42 mmHg (18%) in mean arterial pressure (MAP) without concomitant increase in heart rate. The fall was significant at 20 minutes and maximum at 90 minutes on average. It correlated closely with the initial plasma renin activity (PRA) (r = 0.96) and with the increase in PRA under treatment (r = 0.75). After an 8-day treatment with captopril alone in increasing dosage (mean: 453 +/- 140 mg/day), the fall in MAP still was 20.3% and correlated with that observed with a single dose. Blood pressure returned to normal levels (MAP Less Than 110 mmHg) in seven of these patients. Twelve patients were treated for a mean period of 36 days and all had normal blood pressure at the end of that period. However, seven had required addition of frusemide to the treatment. A moderate increase in serum creatinine was observed in 7 cases, but there were no other adverse reactions. Captopril alone or combined with diuretics proved to be a very effective drug for the treatment of hypertension, particularly in patients with high initial PRA.