Age-related induction of unscheduled DNA synthesis(UDS)by ultraviolet B(UV-B)irradiation was investigated in the epidermis of female hairless mice by means of an in vivo-in vitro assay using a liquid scintillation counting method. Skin samples were taken and cultured in a medium containing[³H]thymidine with or without hydroxyurea(HU)for 2 hr. DNA of the epidermis was extracted, and the incorporation of[³H]thymidine into DNA and the DNA content were determined with a liquid scintillation counter and a fluorescence spectrophotometer, respectively. Induction of UDS by UV-B was judged in terms of the value of the UDS index calculated as a percentage of the respective unexposed control value taken as 100%[the UDS index is given by(the ratio of DNA synthesis in the presence of HU to that in its absence)×100]. DNA synthesis both in the presence and absence of HU decreased with age[12 months old(M)<8M<4M<2M<1M)], concomitantly with a small but significant increase of UDS index. The decrease was high in the younger age groups and moderate in the older age groups. UV-B increased the UDS index approximately 14-, 12- and 9-fold at 1 hr after 1, 000 J/m² irradiation in 1M, 2M and 12M mice, respectively, and these increases were partly reversed at 4 hr after irradiation. UV-B also increased the UDS index approximately 25-, 24- and 21-fold at 1 hr after 4, 000 J/m² irradiation for each age group. However, there was no statistically significant age-related difference in the magnitude of the UDS index after irradiation of UV-B. These results show that replicative DNA synthesis decreases with age, whereas DNA repair capacity after UV-B irradiation does not change with age under these conditions.