-Melanocyte-stimulating hormone (-MSH), atrial natriuretic peptide (ANP), and oxytocin have been identified as candidate hormonal mediators of the reflex natriuresis that follows acute unilateral nephrectomy (AUN). Pharmacological characterization of the third melanocortin receptor (MC3-R) indicates that it uniquely responds to physiological concentrations of -MSH. We tested the roles of -MSH, ANP, and oxytocin in the postnephrectomy natriuresis by carrying out AUN during continuous intrarenal infusion of specific antagonists for their cognate receptors. In anesthetized Sprague-Dawley rats, urinary sodium excretion (U_NaV) increased from 0.34 ± 0.04 to 1.12 ± 0.11 µeq/min 90 min after AUN (P < 0.001). No change in U_NaV occurred in rats undergoing a sham AUN procedure. Plasma immunoreactive -MSH concentration was 53 ± 8 fmol/ml after sham AUN but 112 ± 17 fmol/ml after AUN (P < 0.01). SHU-9119 and SHU-9005 are substituted derivatives of -MSH with potent antagonism at the MC3-R in vitro. Infusion of these compounds at 5 pmol/min completely blocked the natriuretic response to AUN despite a similar elevation in plasma -MSH (111 ± 12 vs. 49 ± 8 fmol/ml in sham rats, P < 0.01). Intrarenal infusion of the ANP receptor antagonist A-71915 (5 pmol/min) or the oxytocin receptor antagonist [d(CH₂)₅¹, Tyr(Me)²,Orn⁸] vasotocin (10 pmol/min) effectively inhibited the natriuresis induced by intravenous infusion of ANP or oxytocin (each at 1 pmol/min), respectively, but did not block the natriuresis after AUN. Plasma immunoreactivity of these peptides was not increased after AUN. These results indicate that reflex natriuresis after AUN is accompanied by an increase in plasma -MSH but not ANP or oxytocin concentration and is prevented by intrarenal infusion of receptor antagonists with selectivity for MC3-R. The data indicate that -MSH or a closely related peptide mediates postnephrectomy natriuresis and provide further support for the possibility that -MSH may play a wider role in sodium homeostasis.