Reactive oxygen species are produced by ultraviolet (UV) exposure and cause oxidative damage. Enzymic antioxidants such as superoxide dismutase (SOD) and catalase (CAT) may play important defensive roles in vivo. The previous studies have focused on the acute effects after single UV irradiation on those enzyme activities. In this study, we investigated the chronic effects of ultraviolet-A (UVA) or ultraviolet-B (UVB) exposure on the skin SOD and CAT activities using hairless mice. Accumulated doses of UVA and UVB after 36-week irradiation conducted 3 times a week were 3240 J/cm2 and 4320 mJ/cm2, respectively. SOD activity was increased by UVB irradiation and gradually returned to control levels, but was unaffected by UVA irradiation. In contrast, CAT activity was suppressed by UVA irradiation, indicating that the skin SOD and CAT activities are not coordinately regulated by long-term UV irradiation. These findings suggest that SOD activity is induced by repeated exposures to UVB in response to chronic photooxidative stress. However, continual cumulative stress may overwhelm the capacity of this system. These time-dependent changes of the cutaneous antioxidant system by chronic UV irradiation should provide us with important information on photooxidative events in cutaneous photoaging.