The effects of stimulating dorsal raphe nucleus (DR) on activities of three deep cerebellar nuclei (DCN) (medial, interposed and lateral nuclei) were investigated. The results were as follows: (1) Stimulation of DR elicited three types of responses, inhibition, excitation, and biphasic response (excitation-inhibition or inhibition-excitation) from DCN cells with responsive latencies ranging from 10 to 84 ms. The majority of responsive cells showed an inhibitory response (76%-90%) with a latency less than 30 ms. (2) The spontaneous discharge frequencies of the DCN cells were 5 to 120 Hz. The cells with higher spontaneous discharge frequencies showed low responsive rates to the DR stimulation, as compared with lower firing frequency cells. (3) The depressive effect of DR stimulation could be blocked by injection of 5-HT2/1c antagonist methysergide (66.7%-83.3%). These results suggest that raphe-cerbellum serotonergic afferent fibers may be involved in the cerebellar sensorimotor integration process by exerting some modulatory effects on the DCN cell's activities.