The firing rate of efferent sympathetic nerves to brown adipose tissue was measured on 18 h or 18 d following lateral hypothalamic lesions (LH). Eighteen hours following acute lateral hypothalamic lesions, sympathetic firing rate was significantly increased. Following chronic LH lesions there was a decrease in food intake and a fall in body weight which had stabilized by four days. Eleven days after surgery a group of control animals were food restricted and subsequently pair fed twice daily to maintain a body weight comparable to that of the LH lesioned animals. Food intake was lower in the pair-gained animals on all but one day of the experiment. When studied 18 days following LH lesions, sympathetic firing rates were significantly higher than in either the ad lib or pair-fed controls. Sympathetic firing rate in pair fed rats, on the other hand, was significantly lower than in the sham lesioned rats. These data are consistent with the hypothesis that the LH lesion removes an inhibitory control over sympathetic firing rate both acutely and in chronically lesioned animals and that this increased sympathetic firing rate may play an important role in the maintenance of a lower body weight.