Expression of coherent and rhythmic circadian (approximate to 24 h) variation of behaviour, metabolism and other physiological processes in mammals is governed by a dominant biological clock located in the hypothalamic suprachiasmatic nuclei (SCN). Photic entrainment of the SCN circadian clock is mediated, in part, by vasoactive intestinal polypeptide (VIP) acting through the VPAC(2) receptor. Here we used mice lacking the VPAC(2) receptor (Vipr2(-/-)) to examine the contribution of this receptor to the electrophysiological actions of VIP on SCN neurons, and to the generation of SCN electrical firing rate rhythms SCN in vitro. Compared with wild-type controls, fewer SCN cells from Vipr2(-/-) mice responded to VIP and the VPAC(2) receptor-selective agonist Ro 25-1553. By contrast, similar proportions of Vipr2(-/-) and wild-type SCN cells responded to gastrin-releasing peptide, arginine vasopressin or N -methyl-d-aspartate. Moreover, VIP-evoked responses from control SCN neurons were attenuated by the selective VPAC(2) receptor antagonist PG 99-465. In firing rate rhythm experiments, the midday peak in activity observed in control SCN cells was lost in Vipr2(-/-) mice. The loss of electrical activity rhythm in Vipr2(-/-) mice was mimicked in control SCN slices by chronic treatment with PG 99-465. These results demonstrate that the VPAC(2) receptor is necessary for the major part of the electrophysiological actions of VIP on SCN cells in vitro, and is of fundamental importance for the rhythmic and coherent expression of circadian rhythms governed by the SCN clock. These findings suggest a novel role of VPAC(2) receptor signalling, and of cell-to-cell communication in general, in the maintenance of core clock function in mammals, impacting on the cellular physiology of SCN neurons.