Captopril was given to 42 hypertensive patients aged 1 to 17 years who were treated by regular haemodialysis because of terminal renal failure. Initially all patients were on antihypertensive treatment without sufficient control of blood pressure. Under captopril all patients presented a significant decrease of both systolic and diastolic blood pressures. The mean decrease of systolic blood pressure was from 162 to 114 and that of diastolic blood pressure from 106 to 86 mmHg. Plasma renin activity rose significantly in all cases whereas plasma aldosterone dropped and reached normal levels for age. The maximum effective dosage was 3 mg/kg/day. Total duration of treatment with captopril ranged from 1 1/2 to 6 1/12 (mean 3 2/12) years. At last observation the dosage of captopril varied between 0.3 and 3 mg/kg/day.