1. Rates of oxygen consumption of albino and hairless mice were measured in a metabolism chamber during periods of approximately 5 or 24 hr. Rectal temperature was measured before and after each period. The chamber temperatures used were 22, 30 and 32 degrees C for both albino and hairless, and in addition 34 and 36 degrees C for the hairless mice.2. The mean age and body weight of the albino mice were 102 days and 34.6 g; the corresponding values for the hairless mice were 87 days and 32.8 g.3. The mean minimum rates of oxygen consumption (ml./kg.min) were 31.0 for the albino and 38.8 for the hairless mouse; the corresponding estimated critical temperatures were in the ranges 30-32 degrees C for the albino mouse and 32-34 degrees C for the hairless mouse.4. The mean values for core-ambient thermal insulation ( degrees C.m(2).hr/kcal) were 0.418 and 0.328 for the albino mouse, and 0.275 and 0.221 for the hairless mouse, at 22 and 30 degrees C respectively in each case.