The acute effects of 50 mg of SQ 14225 (Captopril) on arterial pressure, cardiac output, total peripheral resistance (TPR) and plasma renin activity (PRA) were studied in 14 chronic hemodialysis patients, six of whom were hypertensive. Before treatment, TPR index (TPRI) correlated significantly with mean arterial pressure (MAP) (r = 0.806, P less than 0.01). After Captopril, the reduction of MAP was 10% or more in hypertensive and normotensive subjects at 30 and 120 min. The percentage change in MAP correlated significantly with that in TPRI in all the patients at 30 (r = 0.872, P less than 0.01) and 120 min (r = 0.866, P less than 0.01). There was no correlation between either the changes in cardiac index or the basal values of PRA and the decrease in MAP. The result suggests that vasodilatation was primarily responsible for the fall in blood pressure in chronic hemodialysis patients.