Rabbits received ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD), or sham lesions. These animals were compared on acquisition and reversal of a Pavlovian conditioned discrimination task in which tones were the conditioned stimuli and paraorbital electric shock was the unconditioned stimulus. Eyeblink and heart rate conditioned responses were assessed. Lesions of MD impaired the reversal, but not original acquisition, of the eyeblink discrimination. Heart rate discrimination was somewhat impaired during both acquisition and reversal. There were no differences between lesion and sham animals on control measures assessing general activity, somatomotor shock thresholds, or heart rate unconditioned responses.