ABSTRACT An analysis of the influence of body weight on the energetics of endotherms has been proposed by McNab (1970). It was based on the observations that (1) when body temperature is constant heat production equals heat loss, and (2) heat loss may be described as the product of thermal conductance and the temperature differential between the body and the environment: M = C(Tb–Ta). This expression with some misfortune and inappropriateness has come to be called Newton’s law of cooling. Its application means that the energetics of an endotherm has a limited number of degrees of freedom; at a particular environmental temperature, fixing two of the factors will determine the third. It was argued that thermal conductance and the rate of heat production vary with body size and with ecological conditions. Consequently, these factors are mainly responsible for setting the level of body temperature.