The acute effect of lateral (LH) and medial (MH) hypothalamic lesions on mitochondrial GDP binding in brown adipose tissue (BAT) (an index of thermogenic state) was studied one and two days postlesion. Groups of rats were lesioned, sham-lesioned, or unoperated and were all fasting. An additional group of unoperated rats had access to food throughout the study. The objective was to determine whether the hypermetabolic state and rapid weight loss known to be induced by LH lesions were attributable to the activation of BAT thermogenesis, and, if so, whether these effects were specific for LH lesions. No effect of either lesion on BAT thermogenic state could be detected at either time studied. Despite that fact, LH-lesioned rats, but not MH-lesioned rats, were hyperthermic at both times. We conclude that the prolonged hyperthermia which occurs shortly after LH lesions is not due to an activation of BAT thermogenesis. Instead, it can be likened to the febrile state in which an initial and brief activation of both nonshivering thermogenesis in BAT and shivering thermogenesis in muscles occurs only during the rising phase of the fever and is suppressed as soon as a stable hyperthermic state is reached. It thus appears unlikely that substantial and prolonged activation of BAT thermogenesis is a major mechanism that promotes exaggerated short-term weight loss in the LH-lesioned rat.