Serotonin (5-HT) is involved in the control of behavior. 5-HT can affect cerebellar mossy fibers (mf) discharge during repetitive motor activity. Dysfunctions of the cerebellar serotoninergic system has been linked to ataxia and autism. In anaesthetized rats, air-puff stimulation of the whisker pad evokes local field potentials (LFPs) in the granular layer Crus-IIa. Theta sensory stimulation (TSS: 10 pulses at 4 Hz) usually causes LTD of the evoked LFP, whereas LTP prevails when gabaergic inhibition is blocked. In order to investigate the impact of 5-HT modulation, 5-HT was locally micro-injected either alone or just before TSS. In the former case a transient potentiation occurred (112 ± 33%; n=5; p=0.01 at 15 min after 5-HT application), while in the latter case TSS caused LTP (121 ± 76%; n=5; p=0.04). Interestingly, 5-HT drives Lugaro cells (LCs) to fire thus inhibiting Golgi cells (GoCs) through glycinergic synapses; in turn, GoCs inhibit granule cells (GrCs). In this manner, 5-HT might enhance LTP induction at the mf-GrC synapse through a double inhibition. In fact, TSS after coinjection of 5-HT with strychnine, a glycinergic receptors antagonist, induced LTD (-23 ± 8%; n=5; p=0.005), as in control conditions. These results show that 5-HT is able to modulate the induction of long term synaptic plasticity in the granular layer. Patch-clamp recordings in vitro are currently being performed in order to reveal the mechanisms of 5-HT action.