A comparison is made between the rectal and the body surface temperatures of fed mice housed in individual compartments without bedding while exposed continuously to environmental temperatures of 5° C, 15° C, and 25° C. Surface temperatures of the mice are related to the ambient temperature at which they are held. Rectal temperatures are known to undergo cyclic variations and, except for the first 24 hours at 5° C, are within normal limits throughout a week of exposure. Fasted animals at 5° C cannot maintain a core temperature beyond about 6 to 12 hours and all die within 24 hours. Injection of an LD50 dose of endotoxin fails to depress liver and muscle glycogen and total body carbohydrate after three hours at 15° C, but after an exposure of five hours liver glycogen alone remained unchanged. At 5° C, carbohydrate reserves were depleted in liver, muscle and total body after three hours in fasted mice but not in fed mice. After five hours, muscle glycogen alone was lowered. Endotoxin poisoned mice lost carbohydrates after three hours and five hours at both 5° C and 15° C.