ABSTRACT The interactions of basal rate of metabolism, thermal conductance, body temperature, lower limit of thermoneutrality, and body weight in mammals are compatible with Newton’s law of cooling. A small body weight will normally reduce the level and preciseness of body temperature, but a high basal rate of metabolism or a low thermal conductance may compensate for a small size and permit a high, precise temperature to be maintained. The parameters of energetics that fix the level and preciseness of body temperature in mammals are ultimately correlated in turn with the environmental parameters of climate and food habits. Birds generally have higher temperatures than mammals because the basal rates of metabolism are higher and the conductances lower in birds than in mammals of the same weight.