Two lines of mice have been selectively bred for differential sleep time responses to ethanol. Long sleep (LS) mice sleep over an order of magnitude longer than short sleep (SS) mice. We have found that these behavioral sensitivities are also reflected in the responsiveness of cerebellar Purkinje neurons in those two mouse lines in situ and in intraocular cerebellar brain grafts. The differential sensitivity of Purkinje cells to the depressant effects of ethanol appears to be an intrinsic property of the cerebellum and shows a high genetic correlation with the hypnotic effects of this drug as measured by sleep time. Sleep time studies of neonatally cerebellectomized LS and SS mice indicate that the cerebellum is not the primary determinant of the sensitivity of these mice to the soporific effects of ethanol. The sleep time of SS, but not LS mice, was altered by cerebellectomy suggesting that the cerebellum has different influences on the ethanol-induced loss of righting reflex in these two mouse lines.