The present study examined the role of the medial subnucleus of the medial geniculate (mMG) in classical heart rate (HR) conditioning to a single acoustic conditioned stimulus (CS) in rabbits. Previous electrophysiological and neuroanatomical studies have implicated the mMG as a potential site of plasticity in forming the HR conditioned response (CR) to acoustic stimuli. In addition, several studies have found that bilateral lesions of the rabbit mMG prevent differential conditioning to acoustic stimuli, however animals still exhibit a significant bradycardiac response to the tones. In order to determine if the residual bradycardia seen in differential conditioning studies was due to learned responses or non-associative effects, rabbits with either bilateral electrolytic or ibotenic acid lesions of mMG, and animals with lesions outside of mMG (lesion control), were subjected to one session of single tone Pavlovian conditioning. In this paradigm, an acoustic CS was paired with an aversive unconditioned stimulus (US) in the conditioning groups, and in a pseudoconditioning group the CS and US were unpaired. The results suggest that bilateral lesions of mMG prevent the acquisition of the HR CR relative to control lesioned animals. The results also suggest that cells intrinsic to mMG are involved in conditioned bradycardia to a single tone, as well as in the discrimination between two tones, as reported previously. The lesion effects upon CRs are discussed with respect to other areas in the acoustic thalamus.