Multiple- and single-unit neuronal activities were recorded from cerebellar cortex (Larsell's lobule HVI and adjacent ansiform cortex) and the cerebellar interpositus nucleus during forward (CS-US), backward (US-CS), and explicitly unpaired classical eyeblink conditioning in several rabbits. Whereas learning-related activity was observed in the interpositus nucleus only during forward pairing of the conditioning stimuli, a variety of patterns of learning-related neuronal firings were observed in cerebellar cortex during forward, backward, and even unpaired presentations of the conditioning stimuli. These data suggest that the cerebellar cortex and the deep cerebellar nuclei play different roles during classical eyeblink conditioning.