The study of correlated evolution can lead to new insights about the inheritance patterns of complex traits. In order to better understand the evolution of metabolic rate, we tested whether voluntary activity levels and basal metabolic rate are genetically correlated in 90-wk-old mice (Mus domesticus) from replicated lines of the sixteenth generation of an artificial selection experiment for high early-age wheel-running activity. We measured basal rates of oxygen consumption and carbon dioxide production and also computed the respiratory exchange ratio. Half of the individuals from both selected and control lines had been allowed free access to running wheels since 4 wk of age, while the other half were in standard cages. This design allowed testing of hypotheses about (1) genetic correlations between voluntary activity and metabolic rate and (2) lifetime training effects on metabolic traits. Selection group did not have a significant effect on metabolic traits; therefore, this study does not support some of the implicit assumptions of the aerobic capacity model for the evolution of vertebrate energetics. Activity group also did not affect metabolic rate, indicating that lifetime training does not alter basal metabolism in these mice. However, strong replicate line-within-selection-group differences were detected, indicating the occurrence of random genetic drift. In females, this divergence in metabolic traits attributable to drift was independent of body mass, but in males it was probably caused by a correlated response to selection involving body mass. This study is the first to show such effects of random genetic drift on metabolic traits.