1. In the rodent cerebellum, both presynaptic CB1 cannabinoid receptors and presynaptic mGluR4 metabotropic glutamate receptors acutely depress excitatory synaptic transmission at parallel fibre-Purkinje cell synapses. Using rat cerebellar slices, we have analysed the effects of selective CB1 and mGluR4 agonists on the presynaptic Ca(2+) influx which controls glutamate release at this synapse. 2. Changes in presynaptic Ca(2+) influx were determined with the Ca(2+)-sensitive dyes fluo-4FF AM or fluo-3 AM. Five stimulations delivered at 100 Hz or single stimulations of parallel fibres evoked rapid and reproducible transient increases in presynaptic fluo-4FF or fluo-3 fluorescence, respectively, which decayed to prestimulus levels within a few hundred milliseconds. Bath application of the selective CB1 agonist WIN55,212-2 (1 microM) markedly reduced the peak amplitude of these fluorescence transients. This effect was fully reversed by the selective CB1 antagonist SR141716-A (1 microM). 3. Bath application of the selective mGluR4 agonist L-AP4 (100 microM) also caused a transient decrease in the peak amplitude of the fluorescence transients evoked by parallel fibre stimulation. 4. Bath application of the potassium channel blocker 4-AP (1 mM) totally prevented both the WIN55,212-2- and the L-AP4-induced inhibition of peak fluorescence transients evoked by parallel fibre stimulation. 5. The present study demonstrates that activation of CB1 and mGluR4 receptors inhibits presynaptic Ca(2+) influx evoked by parallel fibre stimulation via the activation of presynaptic K(+) channels, suggesting that the molecular mechanisms underlying this inhibition involve an indirect inhibition of presynaptic voltage-gated Ca(2+) channels rather than their direct inhibition.