The allometric 3/4 power relation, initially used for describing the relation between mammalian basal metabolic rate and body size, is often used as a general model for organismal design. The use of allometric regression as a model has important limitations: it is not mechanistic, it combines all physiological variables into one correlate of body size, and it combines data from several physiological states. In reassessing the use of allometric equations, we first describe problems with their use in studies of organismal design and then use a formulation for distributed net heat production and temperature distribution within the body to derive an alternative equation for the relation between basal metabolism and body size. Tests of the heat flow equation against data reported in the literature indicate that it is an accurate estimator of basal metabolism under thermoneutral conditions and suggest that the allometric equation is a special case of this mechanistic and more general model. We propose that our method is more meaningful and widely applicable for thermoneutral conditions than is a purely allometric approach.