We identified a novel 36-amino acid neuropeptide in rat brain as an endogenous ligand for the G protein-coupled receptors FM-3/GPR66 and FM-4/TGR-1, which were identified to date as the neuromedin U (NMU) receptors, and designated this peptide neuromedin S (NMS) because it was specifically expressed in the suprachiasmatic nucleus (SCN) of the hypothalamus. NMS shared a C-terminal core structure with NMU. NMS mRNA was highly expressed in the central nervous system, spleen and testis. In rat brain, NMS expression was restricted to the ventrolateral portion of the SCN and has a diurnal peak under light/dark cycling, but remains stable under constant darkness. Intracerebroventricular (ICV) administration of NMS in rats induced nonphotic type phase shifts in the circadian rhythm of locomotor activity. ICV injection of NMS also decreased 12-h food intake during the dark period in rats. This anorexigenic effect was more potent than that observed with the same dose of NMU. ICV administration of NMS increased proopiomelanocortin (POMC) mRNA expression in the arcuate nucleus (Arc) and corticotropin-releasing hormone mRNA in the paraventricular nucleus, and induced c-Fos expression in the POMC neurons in the Arc. These findings suggest that NMS is implicated in the regulation of circadian rhythm and feeding behavior.