Aging changes of DNA synthesis and of ultrastructure in mouse colon were analyzed by light and electron microscopic radioautography. The sites of 3H-thymidine uptake were identified in each layer of the colon. The highest labeling index (LI) of the epithelial cells was found at embryonic day 19; it decreased from neonatal day to 1 week and then remained on a constant value until 1 year. The LI's of the other layer cells were high at prenatal day 19, followed by a decrease with the development of the colon disappearing between 2 and 6 months after birth. The LI of the inner circular muscle layer was in every experimental age group higher than that of the outer longitudinal layer. The LI of vacuolated cells, high in the late embryonic and neonatal periods, decreased significantly between 3rd and 8th day and then remained unchanged until the old stage. The LI of the goblet cells showed a peak at prenatal day 19, decreased afterwards with time and disappeared 2 months after birth. The ultrastructural aspects showed that many silver grains located over the nuclei of the labelled cells which contained less cell organelles, in their cytoplasm as compared to unlabelled cells.