Abstract 1. 1. Mice acclimated to ambient temperature ( T A ) 22–24°C are good thermoregulators, when not torpid, at T A 2–34°C. 2. 2. The core-to-subcutaneous temperature gradient varied with T A in a 3. 3. Perognathus longimembris simulated conformation to Newton's Law of Cooling when metabolizing at a minimum maintenance level, but active mice had a variable conductance which decreased with T A . 4. 4. Mice entering and arousing from torpor went through several phases of temperature change. During cooling, conductance is greater than normal, and during warming it is less.