Ethanol (EtOH) triggers cellular adaptations that induce tolerance in many brain areas, including the suprachiasmatic nucleus (SCN), the site of the master circadian clock. EtOH inhibits light-induced phase shifts in the SCN in vivo and glutamate-induced phase shifts in vitro. The in vitro phase shifts develop acute tolerance to EtOH, occurring within minutes of initial exposure, while the in vivo phase shifts exhibit no evidence of chronic tolerance. An intermediate form, rapid tolerance, is not well studied but may predict subsequent chronic tolerance. Here, we investigated rapid tolerance in the SCN clock.