In order to study the aging changes of intramitochondrial DNA synthesis of mouse adrenal cells during the development, 10 groups of developing mice, each consisting of 3 individuals, total 30, from fetal day 19 to postnatal newborn at day 1, 3, 9, 14 and adult at month 1, 2, 6, 12 and 24 were injected with 3H-thymidine, sacrificed 1 hr later and the adrenal tissues were processed for electron microscopic radioautography. On electron microscopic radioautograms obtained from each animal, numbers of mitochondria per cell, numbers of labeled mitochondria and the mitochondrial labeling index labeled with 3H-thymidine showing DNA synthesis in each adrenal cortical cell, in 3 zones, as well as adrenal medullary cells were counted and the results in respective groups were compared, respectively. From the results, it was demonstrated that the numbers of mitochondria in 3 zones, the zona glomerulosa, fasciculata and reticularis of mice at various ages as well as the numbers of mitochondria in 2 types of medullary cells, adrenalin cells and noradrenalin cells, increased from fetal day 19 to postnatal month 1 due to development and aging of animals. On the other hand, the numbers of labeled mitochondria and the labeling indices of intramitochondrial DNA syntheses incorporating 3H-thymidine in 3 zones of cortical cells increased from fetal day 19 to postnatal month 1, while the numbers of labeled mitochondria and the labeling indices of intramitochondrial DNA syntheses in medullary cells, both the adrenalin and the noradrenalin cells incorporating 3H-thymidine increased from fetal day 19 to postnatal day 14 and decreased to month 1. From the results, it was shown that the activity of intramitochnodrial DNA synthesis in both the adrenal cortical and the medullary cells in developing mice changed due to development and aging but the cortical cells and the medullary cells were a little different from each other. The intramitochondrial DNA syntheses in some other organs were also reviewed and discussed.