The purpose of this project is: (1) to investigate age-related changes in thermoregulation, and (2) to examine the physiological mechanisms underlying these changes. We have demonstrated that aged mice have diminished cold tolerance and are not able to adapt to repeated cold exposure. The cause of these aged- related aberrations in thermoregulation appears to be, in part, a reduction in metabolic heat production due to change in brown adipose tissue (BAT) and, in part, a reduction in heat conservation. Efferent sympathetic nervous responses to BAT are enhanced in both cold- acclimated and aged animals and is not changed in animals that "failed" to acclimate. Results indicate that the sympathetic nervous system plays a major role in cold acclimation, but is not responsible for the aged- related decline in thermoregulation. Our result showed that nonshivering thermogenesis is, in fact, enhanced in aged animals. We have shown that exercise training improves metabolic heat production in aged mice and results in weakening of cold tolerance in adult animals.