Backgrounds and Objective: Paraoxon as the active form of parathion is one of the most of organophosphates (OPs) used in agriculture. Some OPs are capable of producing free radicals and inducing disturbance in the body antioxidant systems. The aim of this study was to investigate the effects of paraxaon on oxidative stress induction in the heart and spleen tissues in rats. Materials and Methods: Male Wistar rats were randomly divided into four groups (7 in each groups) as follows: control group received corn oil as paraoxon solvent, and the other three groups received different doses of paraxaon (0.3, 0.7, and 1mg/kg) by intraperitoneal injection. Animals were ether anesthesized 24 hours following the injection, their hearts and spleens were removed, and the levels of superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST) and lactate dehydrogenase (LDH) activities, and glutathione (GSH) and malondialdehyde (MDA) were determined by biochemical methods. Results: Our data indicate that at doses higher than 0.3 mg/kg of paraoxon, SOD and GST levels significantly increases in both the heart and spleen. While there was also an increased CAT activity observed in the heart , both CAT and GSH levels showed a decrease in the spleen. Finally, a decreased LDH activity and increased MDA level in the heart tissue were observed at 1mg/kg dose of paraoxon. Conclusion: Paraoxon induces the production of free radicals and oxidative stress in a dose- dependent manner. The heart tissue appears to be more sensitive to the effects of paraoxon on oxidative stress induction compared to the spleen.