The renal excretion of an intravenous sodium load by ten heart-transplant recipients with (five patients) or without (five patients) moderate renal failure was compared with the results obtained in two matched control groups of ten kidney-transplant recipients with similar renal function. All patients were treated with prednisone and cyclosporine except, for the latter treatment, the kidney-transplant recipients without renal failure. A supplementary control group of five healthy subjects who did not receive any treatment was also included in the study. Fractional sodium and osmolal clearances were greater in heart-transplant than in kidney-transplant recipients for both the patients with and the patients without renal failure. These two parameters were also greater in heart-transplant recipients without renal failure than in healthy subjects. Free water clearance was smaller in the heart-transplant recipients than in the corresponding control patients. Fractional potassium clearance was not different from group to group. These results demonstrate that heart-transplant recipients excrete a greater fraction of the filtered sodium load than their controls. This increased fractional excretion of sodium cannot be attributed to the cyclosporine treatment or the moderate chronic renal failure which both have been taken into account in the comparisons. The unchanged blood pressure during the saline load and the high or normal plasma renin activity levels in the heart-transplant recipients make it also unlikely that the exaggerated natriuresis was the consequence of the higher blood pressure observed in these patients. The chronic cardiac denervation which is the consequence of surgery in these patients could play a role in the mechanism of the saline diuresis.