Spike discharges of single cerebellar Purkinje cells were recorded continuously with extracellular microelectrodes in unanaesthetized curarized rats. The intravenous injection of diphenylhydantoin in doses between 10 and 100 mg kg^–1 did not substantially alter the activity of Purkinje cells within 2–3 h. The two benzodiazepines, diazepam and clonazepam, already in low i.v. doses (0.03–0.1 mg kg^–1) consistently and reversibly depressed the firing rate. Our results do not support the previously advanced hypothesis that these drugs reduce epilepti-form activities by increasing the output from the cerebellar cortex. They rather point to the possibility that a reduced firing rate of cerebellar Purkinje cells mediates at least in part ataxia and muscular hypotonia observed after these drugs.