Rats fasted for 48–96 h before exposure were shown to have a longer survival time at 33 500 ft (1 ft = 0.305 m) simulated altitude than nonfasted controls. Although both groups become hypothermic at 33 500 ft, colonic temperatures of the fasted rats were not sufficiently lower than those of nonfasted animals to explain the difference in survival time. The injection of glucose and insulin before exposure almost completely eliminated the protection afforded by fasting, whereas glucose alone had no effect on survival. It is therefore suggested that an alteration in carbohydrate metabolism, possibly in combination with other starvation-induced changes, allowed fasted rats to survive at 33 500 ft until declining body temperatures reduced metabolic rate to a level compatible with oxygen supply.