Comparisons of the relation between brain and body weights among extant mammals show that brain sizes have not increased as much as body sizes. Interspecific increases in brain and body size appear to occur at the same rate, however, when the amount of available energy is taken into account. After this adjustment, brains of primates are slightly larger than expected from the overall mammalian data, but primates also use a larger proportion of their total energy reserves for their brains. Analyses of relative brain size must take into account the requirements that the metabolically active brain has for the body.