Chlorpyrifos is one of the most commonly used organophosphorus insecticide. Many studies showed a relationship between pesticide exposure and occurrence of diabetes. The aim was to assess toxic effect of chloropyrifos on pancrease, its ability to induce diabetes mellitus in adult male rats and evaluate whether this toxic insult is permanent or temporary. Material and methods: 24 adult male albino rats were classified into four groups. Group I: negative control. Group II: positive control gavaged orally with 0.5 ml of corn oil once a day for 8 weeks. Group III: gavaged orally with chlorpyrifos dissolved in corn oil (6.75 mg/Kg b.w /day) for 8 weeks. Group IV: orally gavaged with chlorpyrifos by the same dose as group III for 8 weeks, then left without treatment for another 4 weeks. At the end of the study, body weight of all rats were measured then blood samples were taken from all rats to estimate serum levels of amylase, lipase, glucose, insulin, reduced glutathione (GSH) and malondialdehyde (MDA) levels. Then the pancreatic tissues were dissected and submitted to histopathological and ultra-structural examination. Results: chloropyrifos administration caused significant increases in serum amylase, lipase, glucose and MDA with also significant decreament in serum insulin and GSH. These biochemical changes were associated with hisopathological changes. Stoppage of exposure for 4 weeks caused partial to complete improvement. Conclusion: chloropyrifos had toxic effects on pancrease and was considered as a risk factor for diabetes mellitus and improvement could occur after stoppage of its exposure for a period.