In this study we aimed to examine how methyl parathion (MP) at sublethal dosages affects 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 19 and 38 millimole (mM) MP dosages as drinking water ad libitum for 28 days continuously. MDA significantly increased in all the tissues except for in the lungs of rats treated with both dosages of MP. With regard to the ADS, SOD significantly decreased in the lungs tissue whereas increased in the erythrocytes with two dosages of MP. GR activity significantly decreased in the erythrocytes treated with both dosages of MP, but decreased in the lungs and liver tissues with 38 mM MP treatment. GST activity significantly elevated in all the tissues except for in the liver treated with 38 mM dosage but did not change with 19 mM. Meanwhile, GSH depletion in all the tissues except lungs of rats treated with both dosages of MP was found to be significant. The observations presented led us to conclude that after the administrations of MP promote MDA content and fluctuate in the ADS in rats. (c) 2008 Elsevier Ltd. All rights reserved.