Brown adipose tissue has been shown to be a strongly thermogenic effector organ in homeothermic animals exposed to cold and in hibernators during cold-induced arousal from deep hibernation. Because of the anatomical distribution of brown fat and the utilization of vascular countercurrent heat exchange, this cold-induced thermogenic response protects the animal by contributing heat to the vital organs of the thorax, the cervical and thoracic segments of the spinal cord, and the sympathetic chain. Evidence indicates that control of thermogenic activity of brown fat is mediated by the sympathetic nervous system.