The present investigation was undertaken to examine the effects of chlorpyrifos (CP, an organophosphate) and deltamethrin (DM, a pyrethroid pesticide) treatments at low and high doses and their combination on lipid peroxidation and antioxidant enzyme activities such as SOD, GSH-Px, and CAT and also AChE activity in rat brain following 16 weeks of exposure. Antioxidative defense mechanisms and lipid peroxidation in rat brain tissues display different responses depending on different pesticide treatments and doses. Biochemical analysis showed that administration of chlorpyrifos and deltamethrin causes brain damage via production of MDA and inhibition of AChE. In the present study, we observed that LPO levels are higher at low doses than at high doses, but DM is more pronounced than CP in this increase. Observations show that SOD increases with the dose, CAT decreases at the higher dose, and GSH-Px again increases with the dose in both pesticide treatments, but especially in DM. Experimentally, oxidative-antioxidative balance is more affected by deltamethrin treatment than that of chlorpyrifos. The effects of combination of DM and CP on LPO may be due to functional, dispositional, or chemical antagonism, while the effect of the combination on CAT activity may be synergistic.