Disrupted circadian rhythmicity is associated with ethanol (EtOH) abuse, yet little is known about how EtOH affects the mammalian circadian clock of the suprachiasmatic nucleus (SCN). Clock timing is regulated by photic and nonphotic inputs to the SCN involving glutamate release from the retinohypothalamic tract and serotonin (5-HT) from the midbrain raphe, respectively. Our recent in vitro studies in the SCN slice revealed that EtOH blocks photic phase-resetting action of glutamate and enhances the nonphotic phase-resetting action of the 5-HT_1A,7 agonist, 8-OH-DPAT. To explore the basis of these effects in the whole animal we used microdialysis to characterize the pharmacokinetics of i.p. injection of EtOH in the hamster SCN extracellular fluid compartment and then studied the effects of such EtOH treatment on photic and serotonergic phase-resetting of the circadian locomotor activity rhythm. Peak EtOH levels (~50 mM) from a 2 g/kg injection occurred within 20-40 min with a half-life of ~3 h. EtOH treatment dose-dependently attenuated photic phase-advances but had no effect on phase-delays, and contrary to in vitro findings, markedly attenuated 8-OH-DPAT-induced phase-advances. In a complementary experiment using reverse-microdialysis to deliver a timed SCN perfusion of EtOH during a phase-advancing light pulse, the phase-advances were blocked similar to systemic EtOH treatment. These results are evidence that acute EtOH significantly affects photic and nonphotic phase-resetting responses critical to circadian clock regulation. Notably, EtOH inhibition of photic signaling is manifest through direct action in the SCN. Such effects could underlie the disruption of circadian rhythmicity associated with alcohol abuse.