Spironolactone was evaluated as a diuretic in the therapy of six patients with chronic severe congestive heart failure, during which metabolic balance studies were performed. Two of the patients showed a clear-cut although moderate diuresis and natriuresis in response to spironolactone in the presence of other diuretics. Two patients had slight unsustained responses and two showed no response. No significant change in potassium excretion was observed. A decrease in the serum sodium level was noted in four patients, associated with a rise in serum potassium. Urinary aldosterone excretion was normal or low in all patients and showed no correlation with the diuretic response to spironolactone.It is concluded that spironolactone has definite but limited usefulness in the therapy of some but not all patients with chronic congestive heart failure.