The hypotensive action of the angiotensin-converting enzyme inhibitor captopril was investigated in 13 hypertensive patients in relation to their age, body weight, the pretreatment level of plasma renin activity (PRA), serum creatinine concentration and suppression of angiotensin II (PA II) by captopril. Captopril was administered in biweekly doubling doses (15 mg, 50 mg and finally 100 mg t.i.d.). The change in systolic blood pressure produced by captopril was significantly (p less than or equal to 0.05 or less) related to age (r = 0.67), to weight (r = 0.55), to the initial PRA levels (r = -0.63) and to the drop in PA II (r = 0.67) but not to the serum creatinine concentration. The change in diastolic blood pressure was also (p less than or equal to 0.05 or less) related to age (r = 0.59), to the pretreatment PRA level (r = -0.71) and to the fall in PA II (r = 0.70) but not to weight or to serum creatinine concentration. Our data suggest that the hypotensive action of captopril is more pronounced in younger hypertensive patients.