Climbing fibers, which originate in the inferior olive and project to Purkinje cells and Golgi cells in the cerebral cortex, were activated at low (0.4-Hz) and high (4-Hz) frequencies by periorbital stimulation in decerebrate ferrets. Climbing fiber responses were recorded as field potentials from the c3 zone of the cerebellar surface. When periorbital stimulation was applied at high frequency, the climbing fiber responses became strongly depressed within a few seconds. It has previously been shown that this high frequency depression (HFD) of climbing fiber responses is due to a cerebellar inhibition of the inferior olive, probably via the nucleo-olivary pathway. Acute administration of ethanol had small and variable effects on the amplitude of climbing fiber responses evoked by low-frequency stimulation. In contrast, medium concentrations (0.44-2.90 g/l) of ethanol led to a marked reduction of the HFD. Low ( < 0.44 g/l) systemic concentrations had no measurable effects on the HFD, whereas high concentrations ( > 2.90 g/l) caused either an increased HFD or a nonseptic reduction in olivary excitability. Because HFD has previously been shown to involve cerebello-olivary inhibition, the possibility of an interaction between ethanol and GABA-ergic responses in the interposito-olivary pathway is discussed.