Introduction & Objective: Diazinon (DZN) is an organophosphate insecticide that one of the mechanisms of toxicity is the inhibition of cholinesterase. The aim of the present study was to evaluate the effects of diazinon on cholinesterase activity in blood serum and erythrocytes of male and female rats and to assess the protective role of vitamin E.

 Materials & Methods: In this experimental study, 60 adult wistar rats including 30 male and 30 female rats were selected and divided into 5 groups (n = 6): control group (without any intervention), sham group (received only pure olive oil daily), experimental group 1 (DZN daily, 60 mg/kg), experimental group 2 (received DZN+ vitamin E daily, with the same dose) and experimental group 3(received vitamin E daily 200 mg/kg). Diazinon and solvent were injected intraperitoneally and vitamin E was given by gavage. After 2 weeks 3 ml blood was taken from the heart tissue, and titrimetric and Ellman’s method respectively were used for serum and erythrocyte cholinesterases activity assay.

 Results: In both genders, due to administration of diazinon, we observed significant reduction in serum and erythrocytes cholinesterase activity. The use of vitamin E increased serum and erythrocytes cholinesterase activity in experimental group 2 of female rats but inhibition in erythrocyte and serum cholinesterase activity was not recovered in experimental group 2 of male rats.

 Conclusion: According to a further reduction of these enzymes activity in female rats with the use of diazinon, it can be concluded that female rats are more sensitive than male rats and it seems that vitamin E as an antioxidants has a protective effect on cholinesterase activity and reduces the toxicity of DZN.