Organophosphorus compounds are widely used in industry, agriculture and for public health purposes. They are among the toxic compounds employed for insect control. The purpose of this work was to study biochemical, histochemical, and histological as well as ultrastructural changes that might occur in the pancreas of adult male Wistar rats as a result of chronic dimethoate intoxication. The treated group received dimethoate orally via gavage (21 mg/kg) daily for 2 months while, the control group was given saline orally (0.1 ml/100 g/day) for the same period. Plasma glucose level was significantly increased while, plasma insulin level was decreased in the intoxicated animals compared with the control group. A patchy reduction of histochemically-detected succinic dehydrogenase enzymatic activity was observed in the pancreas of the intoxicated rats. By contrast, acid phosphatase enzymatic activity was markedly increased in the pancreas of the intoxicated group. No changes were observed in alkaline phosphatase or alpha esterase activities of the intoxicated animals. Light microscopic examination revealed that dimethoate caused patchy degenerative changes of variable severity in many areas of the pancreas affecting both the pancreatic acini and islets of Langerhans. Ultrastructurally, some beta cells revealed dense nuclei with wide perinuclear cisternae. Diminution of the number of beta granules was evident. One month after discontinuation of the dimethoate, all the above mentioned changes induced by dimethoate intoxication persisted. These findings show that chronic exposure to dimethoate insecticide has clear toxic effect on the rat pancreas, which was not reversible within 1 month. Public health education is necessary to raise people awareness about the hazards accompanying the use of such compounds.