The antioxidant effect of CoQ10 on N-nitrosodiethylamine (NDEA)-induced oxidative stress was investigated in mice. Food intake and body weight were similar in both CoQ10 and control groups during the 3-week experimental period. NDEA significantly increased the activities of typical marker enzymes of liver function (AST, ALT and ALP) both in control and CoQ10 groups. However, the increase of plasma aminotransferase activity was significantly reduced in the CoQ10 group. Lipid peroxidation in various tissues, such as heart, lung, liver, kidney, spleen and plasma, was significantly increased by NDEA, but this increase was significantly reduced by 100 mg/kg of CoQ10. Superoxide dismutase activity increased significantly upon NDEA-induced oxidative stress in both the control and CoQ10 groups with the effect being less in the CoQ10 group. Catalase activity decreased significantly in both the control and CoQ10 groups treated with NDEA, again with the effect being less in the CoQ10 group. The lesser effect on superoxide dismutase and catalase in the NDEA-treated CoQ10 group is indicative of the protective effect CoQ10. Thus, CoQ10 can offer useful protection against NDEA-induced oxidative stress.