Each cerebellar Purkinje cell receives input from about 160,000 glutamatergic granule cells. In anatomically intact preparations this input has hitherto been studied only as a compound synaptic potential or current. Presented here are simultaneous recordings in cerebellar slices of synaptically connected granule cell-Purkinje cell pairs. The mean amplitude of the excitatory synaptic currents evoked by stimulation of individual granule cells ranged from 2 to 60 pA, whereas the great majority of the spontaneous glutamatergic currents in Purkinje cells in the presence of tetrodotoxin were < 40 pA. In several cases, stimulation of a single granule cell evoked a disynaptic inhibitory current. It is estimated that on the order of 50 simultaneously active granule cells are sufficient to excite a Purkinje cell.