The acute effects of the converting enzyme inhibitor captopril on blood pressure, heart rate and plasma renin activity were assessed in 53 patients (aged 20-80 years) with mild to moderate essential hypertension under basal conditions. The systolic blood pressure before captopril was elevated in patients in the forties (p less than 0,01) when compared to the younger patients and was positively correlated to the age before (r = 0.32, p less than 0.05) and after (r = 0.32, p less than 0,05) angiotensin converting enzyme inhibition with captopril 25 mg. the drop in systolic (-13%) and diastolic (-10%) blood pressure was significantly greater in patients in the fifties (p less than 0,025 and p less than 0,05 respectively) when compared to younger and older (p less than 0,1) patients. The response of plasma renin activity paralleled the blood pressure response, while heart rate after captopril was not significantly changed in any group. The lack of age dependence of the blood pressure response to captopril and of reflex tachycardia suggests converting enzyme inhibition to be a valuable adjunct for the treatment of essential hypertension in the elderly.