T he γ-melanocyte-stimulating hormone (γ-MSH) is a natriuretic peptide derived from the N-terminal region of proopiomelanocortin (POMC). Evidence suggests that it may be part of the coordinated response to a low-sodium diet (LSD). We tested the effect of the HSD (8% NaCl) compared with LSD (0.07%) on mean arterial pressure (MAP) in mice with targeted disruption of the PC2 gene (PC2^–/–), necessary for processing of POMC into γ-MSH, or the melanocortin receptor 3 gene (Mc3r^–/–; the receptor for MSH). In wild-type mice, HSD for 1 week did not alter MAP versus LSD mice, but plasma γ-MSH immunoreactivity was more than double the LSD value. In contrast, in PC2^–/– mice, MAP on the LSD was not greater than in wild-type mice, but plasma γ-MSH was reduced to one-seventh the wild-type value. On the HSD, MAP rose to a markedly hypertensive level while plasma γ-MSH concentration remained severely depressed. Intravenous infusion of γ-MSH (0.2 pmol/min) for 30 min to PC2^–/– mice after 1 week of HSD lowered MAP from hypertensive levels to normal; infusion of α-MSH at the same rate had no effect. Injection of 60 fmol of γ-MSH into the lateral cerebral ventricle of hypertensive mice also lowered MAP to normal. Administration of a stable analogue of γ-MSH intra-abdominally by microosmotic pump to PC2^–/– mice prevented the development of hypertension when ingesting the HSD. In mice with targeted disruption of the Mc3r gene, the HSD also led to marked hypertension accompanied by elevated plasma levels of γ-MSH; infusion of exogenous γ-MSH to these mice had no effect on MAP. These results strongly suggest that PC2-dependent processing of POMC into γ-MSH is necessary for the normal response to the HSD. γ-MSH deficiency results in marked salt-sensitive hypertension that is rapidly improved with exogenous γ-MSH through a central site of action. α-MSH infused at the same rate had no effect on MAP, indicating that the hypertension is a specific consequence of impaired POMC processing into γ-MSH. Absence of Mc3r produces γ-MSH resistance and hypertension on the HSD. These findings demonstrate a novel pathway mediating salt-sensitivity of blood pressure.