Previous studies have demonstrated that learning-related cerebellar plasticity and stimulus-elicited neuronal activity emerge ontogenetically in parallel with delay eyeblink conditioning in rats. The present study examined cerebellar interpositus field potentials and multiunit neuronal activity evoked by microstimulation of the inferior olive in Postnatal Day 17 and 24 rats. The slope and amplitude of the excitatory postsynaptic potential and the number of evoked multiunit spikes increased with age, whereas the inhibitory postsynaptic potential caused by Purkinje cell input remained stable. These results are consistent with the notion that the postsynaptic depolarization of cerebellar interpositus neurons caused by cerebellar afferents (e.g., the climbing fibers of the inferior olive) is a critical factor contributing to the ontogeny of delay eyeblink conditioning in rats.