Cells exposed to ultraviolet A (UVA) radiation can induce the production of reactive oxygen species (ROS) that may damage cellular elements. By contrast, antioxidants can reduce production of ROS. To assess these cellular events in a model system, rats were divided into three groups comprising control (C), ultraviolet exposed (UV), and ultraviolet exposed and quercetin-treated (UV + Q). UV and UV + Q group rats were irradiated 4 h/day with UVA radiation (1.25 mW/cm(2)) for 9 days. In the UV + Q group rats quercetin (50 mg/kg body weight) was administered intraperitoneally before irradiation. The levels of malondialdehyde (MDA) were increased significantly following irradiation (P < 0.001). In the UV + Q group MDA levels declined significantly compared with the UV group (P < 0.001). With respect to levels of glutathione (GSH), no statistically significant changes were found between the control and the UV group. The GSH levels in the UV + Q group were slightly higher than those of the control and UV groups, but not significantly so. The enzyme activities of glutathione peroxidase, glutathione reductase, catalase and superoxide dismutase decreased significantly after irradiation (P < 0.001). In the UV + Q group all of these enzyme activities were found to be considerably higher than those in the UV group (P < 0.001). This study demonstrates that exposure of rats to UVA leads to oxidative stress as reflected by increased MDA levels and reduced enzymatic antioxidant levels. It also shows that quercetin may be useful in reducing or preventing photobiologic damage.