We have performed a long-term (6-16 months) administration of captopril in 6 hypertensive patients with renal failure undergoing hemodialysis. They were divided into two groups according to the basal renin levels (pretreatment plasma renin activity, PRA), that is, high PRA group (3 patients) and normal PRA group (3 patients). 1) In the high PRA group efficient reduction of blood pressure was obtained by small doses of captopril. In the normal PRA group, however, slight or no pressure reduction was observed. Appropriate initial dose was 10-15 mg/day and maintenance one was 75 mg/day in high PRA group. 2) Hemodialysis potentiated the hypotensive action of captopril in the high PRA group through massive removal of sodium and water. 3) In the normal PRA group PRA was regulated by serum sodium concentration and the plasma aldosterone concentration (PAC) was by serum potassium concentration, but in the high PRA group PRA and PAC were mainly regulated by captopril itself. 4) In one patient an adverse reaction consisting of strong bradycardia, dyspnea, chest pain and cold sweat was observed soon after the start of captopril medication. From these results, it was concluded that captopril was as excellent drug for the control of severe hypertension in dialysed patients having a high basal renin level.