In immature rat cerebellar slices in vitro, a long term depression (LTD) of the responses of Purkinje cells (PCs) to L-glutamate (Glu) was achieved in 30% of the recorded cells by simultaneous stimulation of the neurones by Glu and by climbing fibres (CFs). This effect was not observed for L-aspartate (Asp)-induced responses. Similarly, selective LTD of Glu-induced responses was obtained in 22% of the cells by pairing Glu applications with direct stimulation of the cells which elicited calcium spikes in these neurones. Finally, bath application of phorbol esters also induced a selective LTD of Glu-induced responses in all cells tested. These results suggest that protein kinase C is involved in cerebellar synaptic plasticity.