The effects of lesions of the cerebellum on the acquisition of heart rate (HR) conditioned responses (CRs) were examined in rats. Large lesions of the cerebellar vermis severely attenuated the acquisition of differentially conditioned bradycardic responses in restrained rats without affecting unconditioned HR responses to the tone conditioned stimuli (CSs) or the shock unconditioned stimulus (UCS). In a second experiment, rats were trained unrestrained, and under these conditions the CR was tachycardia in control animals. Lesions of the vermis again severely attenuated acquisition of this CR without affecting unconditioned responses to the CSs or UCS. Bilateral lesions of the cerebellar hemispheres did not affect HR conditioning in either test procedure. It is concluded that the vermis of the cerebellum is an essential component of an HR conditioning circuit in the rat. The cerebellar hemispheres, which are critically involved in some discrete somatomotor CRs, apparently have no essential functional contribution to HR conditioning. The results of these experiments implicating the midline cerebellar vermis in autonomic conditioning are discussed in relation to contributions from a forebrain system involved in HR conditioning and in relation to lateral cerebellar contributions to discrete somatomotor CRs.