Average life spans were estimated for the male progeny from 21 of the 25 possible matings of 5 inbred mouse strains. Oxygen consumption was measured in an open system over a 48-hour interval. Resting metabolism, Mre, and average metabolism, Mav, were determined at 6-8 months of age, and at 24-34 months. Body weight, W, was determined at the time metabolism was measured. Life span, L, is negatively correlated with Mre and Mav, and positively correlated with W at both ages of measurement. This is in accord with the metabolic wear factor that had previously been established among 85 different species of mammals. A new metabolism variable, the energy partition coefficient, defined as the ratio of average to resting metabolic rate, Mav/Mre, has a parabolic relation to body weight, i.e., is maximal at an intermediate body size. The squared body weight deviation in turn has a negative correlation with life span. The correlation of L with Mav/Mre is positive, as expected, but not significant. These data suggest the existence of a longevity factor dependent on the partition of energy between the phasic metabolism of activity and the continuous maintenance metabolism.