The actions of serotonin (5-HT) on synaptic responses in Purkinje cells were examined using the whole-cell voltage-clamp recording from thin slices of the rat cerebellum. Focal stimulation within the molecular layer elicited synaptic responses: (1) inhibitory postsynaptic currents (IPSCs) that were completely blocked by bicuculline; (2) excitatory postsynaptic currents (EPSCs) that were antagonized by CNQX; and (3) combination of IPSC and EPSC. Application of 5-HT (3-30 microM) increased the amplitude of the stimulation-evoked IPSCs but did not significantly affect the EPSCs. The frequency and the mean amplitude of spontaneous IPSCs were increased in the presence of 5-HT. Neither the sensitivity to exogenously applied GABA nor the holding current of Purkinje cells were noticeably altered by 5-HT. The results indicate that 5-HT differentially enhances the GABA-mediated IPSCs in rat cerebellar Purkinje cells via a presynaptic mechanism. Serotonergic neurons thus appear to play a role in the motor control through selective presynaptic facilitation that modulates the output from the cerebellar cortex.