The levels of mitochondrial enzyme activities involved in respiration and oxidative phosphorylation and of specific mitochondrial gene transcripts were examined in rat heart and liver tissues during early growth, development, and aging. Increases were shown in cardiac respiratory complex activities I, III, IV, and V and ATPase6 and CoxII transcript levels during the transition from neonate to young adult. This increased mitochondrial gene expression is not associated with a proportionate increase in mitochondrial number. In contrast, no significant changes in liver mitochondrial activities or transcripts were detected during this transition. Marked reductions in the activities of complexes I, III, IV, and V and in ATPase6 and COXII transcripts were demonstrated in older adult as compared with young adult cardiac tissue with no concomitant reduction in cardiac citrate synthase activity and content, and mtDNA copy number. No decline was noted in liver mitochondrial enzyme activity levels and transcripts of old adult rats. These findings suggest that cardiac mitochondrial gene expression is developmentally regulated at a pretranslational level. The pattern of increasing mitochondrial gene expression in the young adult and decreasing gene expression in the aging heart stands in clear contrast to liver mitochondrial gene expression or nuclear-encoded genes such as citrate synthase.