Rabbits were overtrained using classical conditioning of the rabbit nictitating membrane (NM)/eyelid response. Unilateral electrolytic lesions were then made through electrodes previously implanted in dentate and interpositus cerebellar nuclei ipsilateral to the trained (left) eye. Lesions caused a complete or near-complete abolition of conditioned behavioral responses on the ipsilateral side, but had no effect on unconditioned responses to corneal airpuff. When training was switched to the contralateral (right) side, animals learned within the first few trials, but did not relearn when training was returned to the left (lesioned) side. Control animals in which lesions spared the deep nuclei showed no such learning deficits. Lesions of cerebellar nuclei also abolished conditioned increases in hippocampal CAl neural activity evoked by the tone conditioned stimulus in this paradigm. As with the behavior, training on the right (non-lesioned) side reinstated the conditioned neuronal response within the first few trials of training, even though behavioral responding on lesioned side showed little or no improvement. These results indicate that the cerebellum is an essential structure for the behavioral expression of learning, and plays an important role in the generation of conditioned hippocampal responses observed in this paradigm.