In mammals, as in rats and mice used in the present study, the major internal timekeeping mechanism is located in the suprachiasmatic nucleus (SCN). It is composed of a complex tissue of multiple, individual oscillator cells that drive numerous physiological and endocrine processes via an electrical and humoral output. Several afferent input systems can interact with the clock mechanism and lead to phase-resetting actions. The recent discovery of orexin-containing fibers in the SCN region and the presence of orexin receptors in the SCN prompted us to investigate the possible role of orexin in the SCN. Multielectrode array recordings from dispersed SCN neurons revealed that orexin A dose-dependently enhanced the extracellularly recorded neuronal activity of many neurons (38%), whereas other neurons were inhibited (28%). The influence of orexin A on neuronal activity in the SCN was confirmed by whole-cell patch-clamp recordings from brain slices and dispersed cell cultures. Orexin A caused significant changes in the frequency but not mean amplitude or decay time constant of spontaneous inhibitory postsynaptic currents (sIPSCs). Low concentrations of orexin evoked an increase of sIPSCs, whereas the highest concentration predominantly caused a decrease of sIPSCs. The effects of orexin A on inhibitory postsynaptic currents were prevented by the orexin 1 receptor antagonist SB 334867 and also reduced in the presence of tetrodotoxin. Long-term recordings of the discharge rate of SCN neurons revealed that orexin A is able to induce phase shifts in cultured SCN neurons as well as in organotypic brain slices.