DNA polymerase activities related to DNA repair were examined in the livers of young (6-month-old) and aged (27-month-old) rats irradiated with γ-rays. The activity of DNA polymerase α was little changed in the livers of γ-ray-irradiated rats, while DNA polymerases β and γ were induced in the livers of young and aged rats exposed by γ-ray irradiation. These enzymes were induced from 2 to 6 h after irradiation of young and aged rats, respectively, although the induction in aged rats was weak. DNA polymerase β activity in the livers of young rats irradiated with γ-rays was 2-fold that in aged rats. Similarly, DNA polymerase γ activity in the livers of young rats subjected to γ-ray irradiation was 3-fold that in aged rats. The induction of proliferating cell nuclear antigen (PCNA) in the livers of aged rats irradiated with γ-rays was also delayed compared with young rats. These results indicate that the decline in repair activity in aged rats leads to the accumulation of oxidative damage and DNA mutations in aged tissues.