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For the purpose of clarifying the aging changes of DNA synthesis of mouse mitochondria in adrenal medullary cells, 10 groups of aging mice, each consisting of 3 individuals, total 30, from fetal day 19 to postnatal newborn at day 1, 3, 9, 14, adult at month 1, 2, 6, 12 and 24, were injected with 3H-thymidine, sacrificed 1 hr later and the adrenal tissues were fixed and processed for electron microscopic radioautography. On electron microscopic radioautograms obtained from each animal, number of mitochondria and the number of labeled mitochondria labeled with silver grains due to 3Hthymidine showing DNA synthesis in each adrenalin cell and noradrenalin cell were counted and the results of labeling indices in respective developing groups were compared respectively. From the results, it was demonstrated that the numbers of mitochondria in both adrenalin and noradrenalin cells in the adrenal medullae of mice at various ages increased from fetal day 19 to postnatal month 6 due to aging of animals, respectively, and decreased to month 24, while the number of labeled mitochondria and the labeling indices of intramitochondrial DNA synthesis incorporating 3H-thymidine increased from fetal day 19 to postnatal day 14 (2 weeks), reaching the maxima, and decreased to month 24. It was shown that the activity of intramitochnodrial DNA synthesis in the adrenal medullary cells in aging mice increased and decreased due to aging of animals.