The effects of lesions of the cerebellum on the acquisition and retention of aversive Pavlovian conditioned bradycardia were examined in rabbits. Lesions of the anterior cerebellar vermis severely attenuated the acquisition of simple conditioned bradycardia without disrupting baseline heart rate (HR), or unconditioned HR responses. Also, lesions of the vermis performed after the acquisition of conditioned bradycardia eliminated evidence of prior conditioning. Bilateral lesions of the cerebellar hemispheres did not affect conditioned or unconditioned HR responses. These results were interpreted to indicate that anterior vermis lesions specifically disrupted part of an essential conditioned response pathway without interfering with the neural circuits that mediate unconditioned HR responding. These lesion data, coupled with recent electrophysiological evidence of learning-related changes in neuronal activity within the anterior vermis of the fear-conditioned rabbit, suggest that the anterior cerebellar vermis is critically involved in the acquisition and retention of this rapidly learned autonomic conditioned response.