The activities of DNA polymerases alpha, beta and gamma were determined in mouse liver as a function of age by a combination of glycerol density gradient centrifugation with polymerase specific assays. Although alpha polymerase was preserved throughout the life span, the activity dropped sharply from a high level at the fetal and neonatal stages to a level one order lower after maturation through adjustment of the amount of protein administered. beta polymerase showed similar but less drastic changes than alpha. DNA polymerase gamma activity increased about two-fold in going from newborn to adult stages and remained constant after maturation. According to the amount of DNA, DNA polymerase alpha decreased after birth, but the change was less drastic compared to that through adjustment of the amount of protein. DNA polymerase beta increased the activity 2-3-fold within a period of 3 months following birth. gamma polymerase underwent more than a 10-fold increase in activity through adjustment of the amount of DNA within the same period.