The present study was designed to understand the effects of sublethal dosages of dichlorvos (DIC) and methyl parathion (MP) on acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in various tissues of rats exposed to 5 and 10 ppm of DIC and MP in drinking water ad libitum for 28 days continuously. According to the results, AChE activity was significantly decreased in all the tissues of rats treated with both dosages of DIC and MP except for in the lungs treated with both dosages of DIC. With regard to the BChE, MP caused a significant decrease in the liver, heart and lungs with 5 ppm dosage whereas it did not change the BChE activity in the other tissues with two dosages. Also, DIC caused a significant decrease in BChE activity in the heart tissue treated with both dosages and in the brain of rats treated with 5 ppm. The observations presented led us to conclude that the administrations of MP and DIC at sublethal concentrations inhibited AChE and BChE activities in the rats. These results suggest that inhibition of AChE may be a better biomarker for the assessment of neurotoxic effects in the living. (C) 2009 Elsevier Inc. All rights reserved.