Survival in a confined atmosphere where O-2 is limited may be extended by hypothermia. Rats were placed in a thermoregulated sealed chamber which contained a limited amount of air. Rats were Studied at low ambient temperatures, in which their terminal rectal temperatures (T-B) were 33.2-8.3 degrees C. Rats which had a terminal T-B of 20 degrees C achieved maximal O-2 extraction. Maximal survival time may be expected with an initial thermoneutral temperature followed by hypothermia with a final T-B of 20 degrees C. Resuscitation with no obvious signs of injury was possible after half of the rats succumbed.