Background:The main mechanism of chronic toxicity of diazinon is the oxidative stress induced by free radical production. The present study was designed to investigate the chronic effects of diazinon on oxidative stress markers in brain and heart of rats and the possible protective effects of tert butyl hydroquinone (TBHQ), as an antioxidant. Materials and Methods:Twenty eight male Wistar rats were randomly divided into four groups: diazinon group (10 mg/kg body weight, once a day), TBHQ group (0.028 g/kg of diet, once a day), diazinon + TBHQ group (diazinon, 10 mg/kg body weight, once a day + TBHQ, 0.028 g/kg of diet, once a day) and control group (corn oil, as vehicle of diazinon and TBHQ). The animals were treated with diazinon, TBHQ, or corn oil orally using a stomach tube for 7 weeks. Results:at the end of 7th week and one day after the last oral administration,Total thiol groups, ferric reducing ability of power (FRAP) and malondialdehyde (MDA) levels in brain and heart tissues were investigated. A significant increase in MDA levels (p<0.05) in heart tissue was evident in the diazinon group, when compared to the control group. Rats of the TBHQ and diazinon+TBHQ groups presented a significant increase in thiol groups (p<0.05), when compared to control and diazinon groups. In addition, TBHQ administration significantly increased the FRAP level of brain tissue in TBHQ and TBHQ+diazinon groups, when compared with the control and diazinon groups, respectively (p<0.05). Conclusion:The results of the present study showed that TBHQ treatment could improve antioxidant status in brain and heart tissues of rats with chronic toxicity of diazinon. However, it couldn’t ameliorate the lipid peroxidation sufficiently

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