The suprachiasmatic nuclei (SCN) of the hamster can be maintained for several days in vitro, allowing electrophysiological investigation of the mammalian circadian clock. Application of pituitary adenylate cyclase activating peptide (PACAP) at Zeitgeber time (ZT) 6 on the first day in vitro can phase shift the rhythm of firing rate expressed by SCN neurons on a subsequent day in vitro. Here we report that co-application of neuropeptide Y (NPY) will block the phase-shifting action of PACAP. This blocking action is mimicked by [Leu31,Pro34]NPY and [D-Trp32]NPY but not by NPY(22-36) or avian pancreatic polypeptide. The results indicate that NPY has actions in the SCN via receptors distinct from the Y2 receptor, which mediates the phase-shifting action of NPY.