Diazinon (DZN) is an organophosphate pesticide widely used in agricultural to control insects and in veterinary medicine to control ectoparasites. This study investigated the induction of oxidative stress in the brain, heart, and spleen of Wistar and Norway rats treated with acute doses of DZN. Female Wistar and Norway rats were treated with 25, 50, 100, and 200 mg/kg of DZN by intraperitoneal injection. The animals were sacrificed 24 h after treatment, and tissues were isolated and analyzed. The result of this study shows that DZN at higher doses increased the level of malondialdehyde, superoxide dismutase and glutathione S-transferase activities and decreased glutathione (GSH) level, lactate dehydrogenase, and cholinesterase activities in the brain, heart, and spleen of both rat strains. At these concentrations, DZN toxicity also lead to a significant decrease in catalase (CAT) activity in all tissues of Wistar rat and brain of Norway rat, while it increased heart CAT activity in Norway rat. However, the alteration of these parameters was observed at lower doses of DZN in Wistar rat. These results suggest that DZN at higher doses induces the production of free radicals and oxidative stress in rat tissues and strains by alteration of antioxidant enzyme activity, depletion of GSH, and increasing lipid peroxidation. Induction of oxidative stress in DZN-treated rats is in the order of brain > heart > spleen. Wistar rats appear to be more sensitive to the effects of DZN on oxidative stress induction compared to Norway rat.