The physiological mechanisms that determine basal metabolic rate (BMR) of endotherms have long been debated. This journal has recently presented a new model, based on biophysical principles of heat flux through the tissues of mammals, that attempts to explain the allometric relationship between BMR and body size. We offer a critique of the model and conclude that although the model may describe the physics of heat transfer through the body, it cannot explain mechanistically the level of BMR. BMR determines some of the key variables of the model, but no combination of the variables determines BMR. The model arrives at an equation relating BMR to body mass that is similar to descriptive empirical equations, but this coincidence results from the geometry of the model and the empirical values put into it.