This paper discusses the course of recovery from the thermoregulatory deficits produced in rats by electrolytic lesions in the preoptic/anterior hypothalamic area. Severe damage rendered rats ectothermic, that is unable to maintain their body temperatures at normal levels unless they were incubated at an ambient temperature of 30 degrees C. Less severe damage produced rats that maintained subnormal but stable body temperatures at 23 degrees C, but that did not increase metabolic rate or shiver and whose body temperatures dropped drastically in the cold (5 degrees C). As the animals recovered, nonshivering thermogenesis returned. Eventually the rats became excessively hyperthermic in normal room temperatures, due to very high metabolic rates. They were still unable to shiver or increase metabolic rate further in the cold and were therefore still unable to prevent a large drop in body temperature. Muscle tonus and shivering recovered gradually, and oxygen consumption returned to near normal levels. The data are described in terms of levels of integration of the nervous control of thermoregulation.