Abstract: Objective To observe combined toxicological effect of avermectine and beta-cypermethrin in rats and to explore the mechanism in oxidative damage of the effect.Methods Forty Wistar rats were randomly divided into 4 groups(normal control,avermectine,beta-cypermethring,and avermectine plus beta-cypermethrin group).The rats were treated by gavage for 30 days and their general condition and changes in body weight were observed.By the end of the experiment,the organ coefficients of the rats were calculated and the activities of glutathione peroxidase(GSH-Px)and superoxide dismutase(SOD),and the contents of malondialdehyde(MDA)in serum of the rats were detected.Results Compared with those of single-dose groups,the poisoning symptoms in the avermectine plus beta-cypermethrin group were more severe and prolonged.Compared with the control group,the rats in the three exposure groups had decreased body weight,but only the body weight reduction in male rats was statistically significant(P<0.05).Compared with the control group,the contents of serum MDA significantly increased and the activities of SOD and GSH-PX dramatically reduced(P<0.05)in the three exposure groups.Factorial analysis showed that the combined mode of the effects was additive between avermectine and beta-cypermethrin for the contents of serum MDA(F=0.324,P=0.57) and the activities of SOD (F=2.86,P=0.09)and GSH-Px(F=2.29,P=0.14).Conclusion Combined effect of avermectine and beta-cypermethrin is more toxic than that of single dose exposure and oxidative damage may be one of the mechanisms of the combined toxicological effect.