Diazinon is one of the most widely used organophosphates in agriculture. Toxic effects of diazinon are due to the inhibition of acetylcholinesterase, an enzyme needed for proper nervous system function. This study wits designed to investigate the effects of diazinon at different doses on pancreas and liver tissues and in which dose level diazinon shows its effects. Sixty male Wistar albino rats were included in this study. Animals were initially divided into control and diazinon given groups. There were 10 animals in the control group and 50 animals in diazinon administered group. The latter was divided into five equal subgroups: 25, 50, 100, 200 and 300 mg/kg of diazinon administered groups. Control group was given only saline. All animals in 300 mg/kg diazinon group died. After 24 h, rats were sacrificed under ether anesthesia. Tissue and blood samples were taken for biochemical and histopathological analysis. Sample tissues were examined under light microscope. In biochemical analysis, AST, ALT, LDH, amylase and lipase enzyme activities were measured. One-way ANOVA test was used to compare the groups. In 200 mg/kg diazinon given group, it has been observed some histopathological changes in pancreas and liver tissues. Cholinesterase activities were significantly decreased and alkaline phosphatase levels were increased in all diazinon given groups, when compared with the controls. There was statistically significant difference between the control and diazinon given groups by means of serum amylase, lipase, ALT and AST activities (p < 0.05). LDH activities were significantly increased in 100 and 200 mg diazinon given groups, when compared with the controls (p < 0.05). Histopathological changes were observed only in 200 mg diazinon given group. This evidence suggest that diazinon effect is dose dependent and this is possibly 10-15% of the LD50 dose (200 mg/kg), which cause acute pancreatitis and histopathological changes in liver. (c) 2006 Elsevier Inc. All rights reserved.