Background: Diazinon (DZN) is an organophosphate pesticide commonly used for pest control in agriculture. It may engender a variety of negative effects in non-target species, including humans and animals. The objective of the present study was to evaluate the ameliorative properties of captopril (CAP), as a thiol containing an angiotensin-converting enzyme inhibitor, against DZN-induced oxidative stress. Methods: Twenty-eight male Wistar rats were divided randomly into 4 groups. All the rats were treated orally via gavage once a day for 7 weeks: control (corn oil), CAP (10 mg/kg), DZN (10 mg/kg), and CAP+DZN combination (as mentioned above). Oxidative stress indices in blood serum, liver and kidney homogenates (malondialdehyde [MDA], total thiol groups, and total antioxidant capacity), and erythrocyte hemolysis (superoxide dismutase [SOD] and glutathione peroxidase) were evaluated. Statistical analysis was performed using GraphPad Prism software, version 6.0 (GraphPad, San Diego, CA, USA), by ANOVA, followed by the Tukey post hoc analysis. Results: The MDA content and SOD activity increased significantly in the DZN group compared with those in the control group. Treatment with CAP in the DZN-exposed group significantly decreased (P<0.05) the MDA concentration and the SOD activity. The total thiol groups were decreased in the DZN group and elevated again by CAP treatment. Conclusion: The co-administration of CAP and DZN was able to attenuate lipid peroxidation and enzyme changes caused by DZN.