Natural hibernation is contrasted with induced hypothermia, one of the major differences being that the process of hibernation is physiologically controlled and regulated while hypothermia is a breakdown in temperature regulation causing a weakening or collapse of other homeostatic m.echanisms. As a demonstration of the remarkable control and regulation evinced in natural hibernation, the process of entering the hibernating state is investigated. In hibernation, the process is passive, wjth declines in respiratory rate, heart rate, and oxygen consumption preceding the drop in body temperature; in enforced hypothermia the animal chills in spite of a violent metabolic effort to remain warm. This control is demonstrated throughout hibernation; between ambient temperatures of about 4° and 15°C, the animal passively follows the environment, but if the temperature goes below 0°C the metabolic rate is increased and body temperature is maintained above freezing. The circulatory process in the aro)lsing hibernator is followed. The arousing hibernator1 s circulatory control and ability to rewarm are in strong contrast to the animal undergoing induced hypothermia; in hypothermia almost all ability to rewarm is lost. It is shown that the hibernators have a specialization of temperature control which is unique among the vast array of mammals. However, the precise nature of the changes made during this resetting of the "physiological thermostat" is largely a mystery.