By immunohistochemical analysis, we detected a gut-brain peptide, neuromedin U (NMU), in the suprachiasmatic nucleus (SCN) which is the site of master circadian oscillator. The NMU mRNA expressed circadian rhythm with the peak at CT4-8 in the SCN. The two NMU-binding receptors (NMU-R1 and NMU-R2) were also expressed in the SCN, but phase angles were different between both receptors. Intracerebroventricular injection (ICV) of NMU induced Fos protein in the SCN cells and caused a phase dependent phase shift of the circadian locomotor activity rhythm. The magnitude of phase shift was dose-dependent. This NMU-induced phase-shift was non-photic type. RT-PCR analyses revealed increases in the expression of immediate early genes, such as c-fos, NGFI-A, NGFI-B and JunB in SCN. Furthermore, ICV injection of NMU increased the expression of Per1, but not Per2 in the SCN. These results indicate that NMU may play some important role for circadian oscillator by autocrine or paracrine action in SCN.