The study of neural arousal mechanisms has been greatly aided by the discovery of the orexin peptides (orexin A and orexin B), the subsequent identification of the neurons that synthesize these peptides, their projections in the brain, and the distribution of orexin receptors in the central nervous system. Orexin neuron activation is partly controlled by circadian signals generated in the brain's main circadian pacemaker, the suprachiasmatic nuclei (SCN). The SCN clock is in turn reset by arousal-promoting stimuli and, intriguingly, orexin fibers and receptor expression are detected in the SCN region. It is unclear, however, if orexin can alter SCN neuronal activity. Here using a coronal brain slice preparation, we found that orexin A and orexin B (0.1-1 microM) elicited significant changes in the extracellularly recorded firing rate and firing pattern in approximately 80% of rat SCN cells tested; the most common response was suppression of firing rate. Co-application of orexin A with a cocktail of ionotropic GABA and glutamate receptor antagonists did not alter the actions of this peptide on firing rate, but did change some its effects on firing pattern. We conclude that orexins can alter SCN neurophysiology and may influence the transmission of information through the SCN to other CNS regions.