We determined skeletal muscle, heart and liver mitochondrial electron transport activities in rats and dogs of various ages. In the skeletal muscle mitochondria, decrease in the activity of complex I was observed in rats aged 28 weeks, and further reduction of the activity was observed in rats aged 55 weeks. A significant decrease in complex IV activity was observed in rats aged 55 weeks. No significant reduction in complex II and III activities were observed in rats aged up to 100 weeks. Significant decreases in complex I and IV activities were observed in heart muscles of rats aged 100 weeks, while no significant changes in the activity of complex I in liver mitochondria were observed in rats aged up to 100 weeks. Similar results were obtained in dogs, i.e., the activity of complex I was the most susceptible to aging among the activities of complexes; and skeletal muscle mitochondria were the most susceptible to aging among the tissues. From our results, involvement of mitochondria in the development of age-related decline in cellular function is especially emphasized in post mitotic cells, and age-associated mitochondrial functional changes are stressed in mitochondrial complexes which contain mitochondrial DNA-encoded subunits.