Angiotensin converting enzyme inhibitors can improve haemodynamics in patients with congestive heart failure and may enhance sodium excretion in hypertensive patients. In a metabolic unit we assessed the effects of one of these agents on renal function in nine patients with stable New York Heart Association functional class 3 or 4 congestive heart failure. Single blinded, the patients received placebo for three days, 25 to 100 mg of captopril three times a day for three days, and three more days of placebo. Mean blood pressure decreased during captopril, with little change in heart rate or respiration. Serum urea was slightly higher during captopril administration. The mean change in creatinine clearance during captopril was insignificant, but it decreased more than 25% in three of nine patients. Decreases in creatinine clearance correlated with lower blood pressure during captopril and were most obvious in patients with high baseline plasma renin activity. Urine output and both sodium and potassium excretion decreased during captopril. Thus captopril failed to improve natriuresis in patients with congestive heart failure and close monitoring of kidney function is necessary when using this agent in patients with congestive heart failure, particularly when blood pressure falls to lower levels.