To explore the mechanism underlying the natriuretic effect of gamma-melanocyte-stimulating hormone (gamma-MSH), we infused the peptide intravenously at 200 pmol/min into anesthetized rats. gamma-MSH led to a progressive increase in urinary sodium excretion (UNaV), whereas continuous infusion of the vehicle did not affect UNaV. Plasma immunoreactive gamma-MSH was nine times greater at 120 min after the start of the infusion than in vehicle-infused rats. Plasma atrial natriuretic peptide (ANP) concentration also increased as a consequence of the gamma-MSH infusion, and a strong correlation existed between the concentrations of the two peptides (n = 17, r = 0.81, P < 0.001). Urinary excretion of guanosine 3',5'-cyclic monophosphate and adenosine 3',5'-cyclic monophosphate increased as a result of the infusion. Antiserum to rat ANP blunted the natriuresis only slightly, suggesting that the increase in plasma ANP concentration was not a critical element in gamma-MSH natriuresis. gamma-MSH had no effect on ANP release from isolated rat right atrial strips superfused in vitro. Infusion of gamma-MSH (500 fmol/min) directly into one renal artery led to an ipsilateral natriuresis without change in UNaV from the contralateral kidney. Prior denervation of the infused kidney prevented the natriuresis resulting from intrarenal infusion. Intrarenal infusion of ANP (800 fmol/min) also produced ipsilateral natriuresis, which, however, was not affected by renal denervation. These studies confirm that the natriuretic action of gamma-MSH occurs primarily by an interaction with the renal nerves. Intravenous infusion of the peptide sufficient to produce a supraphysiological plasma gamma-MSH concentration also results in an increase in plasma ANP concentration; however, this increase at best plays only a minor role in the natriuresis following intravenous gamma-MSH infusion.