The effect of synthetic melanocyte-stimulating hormones (MSH) on urinary excretion of sodiu, potassium and water was studied in hamsters. Synthetic alpha-MSH and synthetic human beta-MSH showed a marked natriuretic and diuretic effect depending on the dosage of hormone. The natriuretic effect of both hormones was approximately equal and did not parallel the magnitude of the melanocyte-stimulating activity of the investigated peptides. Both peptides showed also a milder kaliuretic effect independent on their dose. The rise of the sodium/potassium index in urine was significant after both peptides. Bilateral adrenalectomy increased further the natriuresis induced by alpha-MSH, while it did not affect the effect o MSH on diuresis and kaliuresis. Changes in urinary excretion of sodium and potassium are considered to be consequence of direct renal action of MSH.