After irradiating mouse epidermis in vivo with ultraviolet radiation (UV), autoradiography using 3H-thymidine was performed to study the effects of UV on scheduled and unscheduled DNA synthesis during a 7 day-period. Suppression of scheduled DNA synthesis by 100 mJ/cm2 of UVB was continued for about 24 h and induction of unscheduled DNA synthesis by 100 mJ/cm2 of UVB for about 6 h after irradiation. It was also confirmed that UVA induced DNA damage if a dose of UVA over 30 J/cm2 was used. When UVA was applied 1 h before UVB irradiation, a significant increase in sparsely labeled cells (SLC) was observed and the numbers of SLC increased in proportion to the total dosage of UV. So it was confirmed that UVA preirradiation enhanced DNA damage by UVB.