The recent increase of ultraviolet (UV) rays on Earth due to the increasing size of the ozone hole is suggested to be harmful to life and to accelerate premature photoaging of the skin. The detrimental effects of UV radiation on the skin are associated with the generation of reactive oxygen species (ROS) such as superoxide anion radical (O-.(2)-) hydrogen peroxide (H2O2), hydroxyl radical ((OH)-O-.), and singlet oxygen (O-1(2)). However, direct proof of such ROS produced in the skin under UV irradiation has been elusive. In this study, we report first in vivo detection and imaging of the generated ROS in the skin of live mice following WA irradiation, in which both a sensitive and specific chemiluminescence probe (CLA) and an ultralow-light-imaging apparatus with a CCD camera were used, In addition, we found that O-.(2)- is formed spontaneously and O-1(2) is generated in the UVA-irradiated skin. This method should be useful not only for noninvasive investigation of the spatial distribution and quantitative determination of ROS in the skin of live animals, but also for in vivo evaluation of the protective ability of free radical scavengers and antioxidants,