Oxygen consumption is markedly elevated in rats with lesions of the lateral hypothalamus (LH), a response typically associated with hyperactivity. To test for involvement of adrenergic systems in this hypermetabolic state, propranolol was administered before and immediately after LH lesions. Propranolol attenuated both the lesion-induced rise in oxygen consumption and activity in the 12 h after surgery. A second experiment evaluated the contribution of activity to this thermic response by lesioning rats immobilized by a continuous barbiturate infusion. The persistence of lesion-induced increases in oxygen consumption in the absence of activity and the attenuation of this response by propranolol demonstrated that 1) LH lesions directly alter metabolic heat production, and 2) this effect is at least partly mediated by beta-adrenergic systems. The functional significance of increased energy expenditure after LH lesions is discussed in light of the known effects of this lesion on the level of regulated body weight.