Background：Ultraviolet irradiation causes various changes in cells or tissues including DNA damage, which results in changes of cell cycling, mutation and cell death such as apoptosis or necrosis. p53 has been studied widely to be as a regulator gene to modulate cell cycling. Previous report shows that RAD50 is a gene to be associated with p53 to repair damaged DNA. Objective and method：In this study, we evaluated changes of RAD50 and p53 expression by ultraviolet B (UVB) irradiation in rat skin in vivo and in human fibroblasts (hF) in vitro. Results：In rat skin irradiated with 400mJ/cm2 UVB, RAD50 protein and mRNA expression were decreased by from early stage after irradiation, and they were restored to its normal level after 12 h. In hF irradiated with 20mJ/cm2 UVB, change of RAD50 expression by UVB irradiation was similar to that of rat skin. On the contrary, p53 protein expression was increased by UVB irradiation from 6 h and 3 h after UVB irradiation in rat skin and hF, respectively, but p53 mRNA expression was not changed by UVB irradiation. Conclusion：RAD50 and p53 expression is modulated differently in UVB- irradiated skin. Further studies are warrented to evaluate functional relationship between the genes in repairing damaged DNA. (Korean J Dermatol 2000;38(2)：213~220)