Organophosphate (OP) pesticides such as dichlorvos (DDVP) intoxication has been shown to produce oxidative stress due to the generation of free radicals, which alter the antioxidant defense system in erythrocytes. In this study, the eff ects of DDVP (1, 10, 100 μM) or DDVP + vitamin C (VC; 10 μM) or vitamin E (VE; 30 μM), on the levels of malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) activities in human erythrocytes were examined in vitro. There were no statistical diff erences between all groups for 1 μM concentration of DDVP. Treatment with DDVP alone produced an increase in the level of MDA and decreased activities of antioxidant enzymes (P < 0.05). Groups treated with vitamins and DDVP showed protective eff ects of vitamins against DDVP-induced changes in antioxidant enzyme activity and lipid peroxidation (LPO) (10 μM). At 100 μM concentration of DDVP vitamins had no eff ect on DDVP-induced toxicity. The results show that administration of DDVP resulted in the induction of erythrocyte LPO and alterations in antioxidant enzyme activities, suggesting that reactive oxygen species (ROS) may be involved in the toxic eff ects of DDVP. Also the data show that the plasma level of VC and VE may ameliorate OP-induced oxidative stress by decreasing LPO in erythrocytes at certain doses of OP pesicides.