In a previous paper it was reported that haloperidol plus morphine produces a marked hypothermia, whereas each drug alone had only negligible effects on temperature. A large hypothalamic lesion produced variable hypothermia lasting several days. After the lesion, rats could be classified in two groups: A--those which had marked decreases of colonic temperature (from 3 to 7 degrees C) and B--those which had a slight decrease (about 1 degree C). The administration of haloperidol, which in normal rats has no effect on temperature, elicited in hypothalamic-lesion rats, whether or not they were already hypothermic, a marked decrease in temperature lasting several hours. Rats of group B showed a faster decrease as well as a faster recovery of temperature than those of group A. It was concluded that the hypothalamic lesions produced an effect equivalent to an increase in opioid activity and/or a decrease in dopaminergic activity.