Abstract: ObjectiveTo study the effect of organophosphorus pesticide,O-methyl-O-p-methiophenyl thiophosphoramidots (MMT)on acetylcholinesterase(AChE)activities of rat blood and human erythrocyte membrane.Methods: Ellman method and Hydrox yamine method were used to measure AChE activity.ResultsAfter rats treated with MMT at the dose of 6.81mg/kg/day for 5 days or treated with MMT at the doses from 0.05 to 1.00mg/kg/day for 90 days,their blood AChE activities decreased obviously;In vitro,MMT could irreversibly inhibit AChE activity on human erythrocyte membrane in dose and t im-edependent manners.ConculutionThese results show that,similar to other or ganophosphorus pesticides,inhibiting AChE actvity is major toxic mechanism of MMT.