The present study examined the role of the auditory cortex in the extinction of differentially conditioned heart rate (HR) responses in rabbits. Lesions were placed bilaterally in either the auditory cortex or the visual cortex. Three days after recovery from surgery, the auditory cortex lesion group and the visual cortex lesion control group were habituated to the tonal conditioned stimuli (CSs), and then given 2 days of Pavlovian differential conditioning (60 trials per day) in which one tone (CS+) was always paired with the unconditioned stimulus and another tone (CS-) was never paired with the unconditioned stimulus. Animals that had demonstrated reliable differential conditioning (CS+ response at least 5 beats greater than the CS- response) were placed on an extinction schedule for 7 days. The extinction schedule was identical to the differential conditioning schedule with the exception that shock never followed the CS+. The results of the study indicate that auditory cortex lesions prevent the extinction of differential bradycardia conditioned responses (CRs) to tonal CSs. Whereas the bradycardia responses to the CS+ quickly extinguished in the group that had control lesions in the visual cortex, the auditory cortex lesion group continued to exhibit significantly larger bradycardiac HR CRs to the CS+ relative to the CS- during all 7 days of extinction. These results suggest that the animals in the auditory cortex lesioned group did not inhibit responses to a previously reinforced stimulus (i.e., CS+) as well as animals with control lesions in the visual cortex.