Pesticides such as organophosphorus and organochlorine compounds commonly used in agriculture for achieving better quality products are toxic substances and lead to generation of reactive oxygen species (ROS) which have harmful effects on human health. While pyrethroid pesticides are used in preference to organophosphates and organochlorines due to their high effectiveness, low toxicity to non-target organisms and easy biodegradability, they may also produce oxidative stress. Thus, we investigated the effects of chlorpyrifos (CP, an organophosphate) and deltamethrin (DM, a pyrethroid pesticide) treatments at low and high doses on lipid peroxidation (LPO) and antioxidant enzyme activities such as SOD, GSH-Px and CAT in rat liver following 16 weeks exposure. Antioxidative defence mechanisms and lipid peroxidation in rat liver tissues display different responses depending on different pesticide treatments and doses. Biochemical analysis showed that administrations of the chlorpyrifos and deltamethrin cause liver damage. In the present study, we observed that lipid peroxidation levels are higher at high doses than at low doses, but DM caused more pronounced increase than CP. Experimentally, we have also observed that oxidant-antioxidant balance is more affected by deltamethrin treatment than by chlorpyrifos. Copyright (C) 2007 John Wiley & Sons, Ltd.