In this study, the effects of dichlorvos (DIC) at sublethal concentration on malondialdehyde (MDA) content and antioxidant defense system (ADS) such as reduced glutathione (GSH), glutathione reductase (GR), superoxide dismutase (SOD), and glutathione-S-transferase (GST) in various tissues of rats exposed to 0.0225 and 0.0450 millimole (mmol) DIC in drinking water were administered orally to rats ad libitum for 28 days continuously. MDA significantly increased in all the tissues except in the lungs of rats treated with both dosages of DIC. With regard to the ADS, GR significantly decreased in the erythrocytes whereas it did not change in the other tissues with two dosages of DIC. SOD activity significantly increased in the all tissues treated with both dosages of DIC. GST activity significantly increased in all the tissues except in the erythrocytes, spleen, and lungs treated with 0.0225mmol. Meanwhile, GSH depletion in the liver and heart of rats treated with 0.0225 mmol and all the tissues except in the brain and kidney of rats treated with 0.0450 mmol of DIC were found to be significant. The observations presented led us to conclude that the administrations of DIC promote MDA content and fluctuate in the ADS in rats during the exposure. (c) 2008 Elsevier Inc. All rights reserved.