Allometric principles account for most of the observed variation in maximum life span among mammals. When body-size effects are controlled for, most of the residual variance in mammalian life span can be explained by variations in brain size, metabolic rate and body temperature. It is shown that species with large brains for a given body size and metabolic rate, such as anthropoid primates, also have long maximum life spans. Conversely, mammals with relatively high metabolic rates and low levels of encephalization, as in most insectivores and rodents, tend to have short life spans. The hypothesis is put forward that encephalization and metabolic rate, which may govern other life history traits, such as growth and reproduction, are the primary determinants directing the evolution of mammalian longevity.