We examined age-associated changes of respiratory enzyme activities and protein synthesis in mitochondria isolated from mouse brain with high oxidative activities. Cytochrome c oxidase activity increased unexpectedly with aging, while the mitochondrial translational activities showed two phases of alterations: they increased progressively up to 21 weeks after birth followed by a gradual decrease with aging. Results showed that these changes were not due to the change in mitochondrial DNA (mtDNA) copy number or the accumulation of deletion mutations in mtDNA. These observations suggest that the common feature of age-associated changes in both human and mouse mitochondrial functions is limited to the decrease in mitochondrial translational activity. Therefore, mouse brain can be used as a model to understand the relationships between aging and mitochondrial function by examining the cause of decrease in mitochondrial translation activity.