Deltamethrin is a alpha-cyano pyrethroid insecticide used extensively in pest control. Although initially thought to be least toxic, a number of recent reports showed its toxicity in mammalian and non-mammalian laboratory and wildlife animal species. In fish, it is a potent oxidative stress-inducing agent. We studied the oxidative stress-inducing effect of deltamethrin in mice.Male Swiss albino mice were orally administered 2 doses of deltamethrin viz., 5.6 and 18mg/kg body weight (bw), for 15 days.Both the doses of deltamethrin significantly induced lipid peroxidation (LPO) in liver and kidney. Along with the induction of LPO, activities of vital antioxidant enzymes such as glutathione peroxidase (GPx), glutathione S-transferase (GST) and catalase (CAT) were also suppressed in both the tissues. Glutathione (GSH) level was also decreased. GSH decrease was more pronounced in kidney than the liver.Toxicity of many chlorinated and organophosphate insecticides is mediated by the reactive oxygen species (ROS). Findings of the present investigation also suggest a role for ROS in deltamethrin toxicity. An increased LPO indicates that these ROS might have caused degradation of biomembrane in deltamethrin-exposed animals.