The physiological role of mGluR1-induced inward current in a Purkinje neuron was studied in a cerebellar slice preparation. In a voltage-clamp condition, application of a mGluR1 agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) induced both the transient and sustained phases of inward currents, which were blocked by simultaneous application of a mGluR1 antagonist 7-(hydroxyimino)cyclopropa(b)chromen-1a-carboxylate ethyl ester (CPCCOEt). Application of CPCCOEt alone reduced the holding inward current, suggesting that a subset of mGluRls is activated in a resting condition. In a current-clamp condition, DHPG increased the frequency of action potentials, whereas CPCCOEt decreased it. These results suggest that the basal mGluR1 activity contributes to the action potential firing by maintaining the sustained inward current in a Purkinje neuron.