1. CB-1 cannabinoid receptors are strongly expressed in the molecular layer of the cerebellar cortex. We have analysed, in patch-clamped Purkinje cells (PCs) in rat cerebellar slices, the effect of the selective CB-1 agonists WIN55,212-2 and CP55,940 and of the selective CB-1 antagonist SR141716-A on excitatory synaptic transmission and synaptic plasticity. 2. Bath application of both agonists markedly depressed parallel fibre (PF) EPSCs. This effect was reversed by SR141716-A. In contrast, responses of PCs to ionophoretic application of glutamate were not affected by WIN55, 212-2. 3. The coefficient of variation and the paired-pulse facilitation of these PF-mediated EPSCs increased in the presence of WIN55,212-2. 4. WIN55,212-2 decreased the frequency of miniature EPSCs and of asynchronous synaptic events evoked in the presence of strontium in the bath, but did not affect their amplitude. 5. WIN55, 212-2 did not change the excitability of PFs. 6. WIN55,212-2 impaired long-term depression induced by pairing protocols in PCs. This effect was antagonized by SR141716-A. The same impairment of LTD was produced by 2-chloroadenosine, a compound that decreases the probability of release of glutamate at PF-PC synapses. 7. The present study demonstrates that cannabinoids inhibit synaptic transmission at PF-PC synapses by decreasing the probability of release of glutamate, and thereby impair LTD. These two effects might represent a plausible cellular mechanism underlying cerebellar dysfunction caused by cannabinoids.