Ultraviolet A (UVA) light exposed cells can induce the production of reactive oxygen species (ROS) which can damage the cellular elements. Antioxidants call interfere with the production of ROS. In this study, malondialdehyde (MDA), reduced glutathione (GSH), glutathione reductase (GSSGR), glutathione peroxidase (GPx), catalase (CAT) and superoxide dismutase (SOD) levels were measured in the liver of rats exposed to UVA light in various doses. The effects of quercetin were determined as antioxidant on those parameters. Rats were divided into three groups as control, ultraviolet (UV), and ultraviolet + quercetin (UV + Q). UV and UV + Q group rats were irradiated 4 h per day with UVA light (1.25 mW/cm(2)) during periods of 0, 3, 6, 9 days. Thus, on days 0: 3, 6 and 9, the rats have received 0, 54, 108, 167 W/cm(2) light, respectively. Quercetin (50 mg/kg body wt.) was administered intraperitoneally before each irradiation period in the UV + Q group rats. MDA level in the UV group increased significantly on day-9 when compared to the control group (P < 0.05). The MDA levels in the UV + Q group decreased significantly on day-6 and 9 in comparison with the UV group (P ( 0.05, P < 0.001, respectively). GSH levels in all groups were not significantly different. GSSGR and GPx activities in the UV group were significantly lower on day-6 and 9 than in the control group (P < 0.001). On all days these enzyme activities in the UV + Q group were significantly higher than in the UV group and higher than in the control group (P < 0.001). SOD and CAT activities in the UV group decreased significantly on day-3, 6, and 9 in comparison with the control group (P < 0.001). These enzyme activities also increased significantly in the UV + Q group on all days when compared to the UV group (P < 0.001). This study demonstrated that the exposure of rats to UVA led to oxidative stress as reflected by increased MDA levels and reduced enzymic antioxidant levels, quercetin may be useful by reducing or preventing photobiologic damage. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.