[en] The present in viro study deals with determining the relation between the concentration of the pesticide methamidophos and the activity of the neurotration acetylcholine esterase (AChE) in brain and erythrocytes of rats and also the concentration of the pesticide that cause 50% inhibition of the enzymes. The in vivo study deals with the effect of methamidophops on the female sex hormones, estradiol and progesterone, following five successive oral treatments with 10mg kg week. The in vitro relation between pesticide concentrations and AChE activities were studied by preincubating different concentration of methamidophos with rat brain homogenates and hemolyzed erythrocytes for different periods. The data showed that AChE activity was clearly decreased. The interaction may be considered irreversible inhibition because it finally leads to a complete block of the enzyme activity that clarified by permanent inhibition. The concentration of the pesticide that cause 50% inhibition for brain and erythrocytes-AChE were found to be 3.2x10^-5M and 10^-4M respectively, following 30 min preincubation period