Abstract: The effects of stimulating dorsal raphe nucleus (DR) on activity of cerebellar interposed nucleus (IN) was investigated.Stimulation of DR elicited inhibitory,excitatory and biphasic (inhibition-excitation and excitation-inhibition) responses from IN cells.The majority of responsive cells showed an inhibitory response (76.0%) with a latency of less than 30 ms.The spontaneous discharge frequencies of the IN cells were 5 to 120 Hz,and the IN cells with higher discharge frequencies showed low responsive rates to the DR stimulation in comparison with those lower firing frequency cells.Injection of 5-HT 2/1C receptor antagonist methysergide (iv) could block the depressive effect of DR stimulation on the IN cells (85.7%).These results suggest that raphe-cerebellar serotonergic afferent fibers may be involved in the cerebellar sensorimotor integration process by exerting their modulatory action on the deep cerebellar nuclear cells activities.