Diazinon (DZN) is an organophosphate insecticide has been used in agriculture and domestic for several years. Vitamin E (200 mg/kg, twice a week), diazinon (10 mg/kg, per day) and Vitamin E (200 mg/kg, twice a week) + diazinon (10 mg/kg, per day) combination were given to rats orally via gavage for 7 weeks. Pseudocholinesterase in serum and haematological indices were investigated at the end of the 1st, 4th and 7th weeks comparatively with control group. At the end of 1st, 4th and 7th weeks, statistically significant decrease of pseudocholinesterase activity in serum were detected when diazinon- and Vitamin E + diazinon-treated groups compared to control group. When diazinon- and Vitamin E + diazinon-treated groups were compared to each other there were no significant changes. When diazinon-treated group was compared to control group, body weight decreased significantly at the end of the 4th and 7th weeks. It was observed that at the end of 1st, 4th and 7th weeks, there was a statistically significance in haematological indices except mean corpuscular hemoglobin (MCH) when diazinon-treated group was compared to control group. At the end of 1st week increase of thrombocyte, at the end of the 4th week increase of hemoglobin and thrombocyte and at the end of the 7th week increase of red blood cell (RBC), hemoglobin, hematocrit, mean corpuscular hemoglobin concentration (MCHC) and thrombocyte were observed statistically significant when Vitamin E+diazinon treated group was compared with diazinon treated group. According to the present study, we conclude that Vitamin E reduces diazinon toxicity, but it does not protect completely. (c) 2005 Elsevier B.V. All rights reserved.