Methamidophos (Me) and its N-acetylated derivative, acephate (Ac), are water soluble insecticides that have similar insecticidal potency, but different mammalian toxicity. Me is a potent inhibitor, while Ac is a poor inhibitor of mammalian AChE (mAChE). At physiological pH, both insecticides exhibit similar accumulation in RBC, while Ac exhibits greater binding to plasma proteins than Me. These differential effects of Ac and Me are attributed to the differences in their physicochemical, molecular-orbital and electronic properties. Ac and Me are freely soluble in aqueous solution, moderately soluble in ethyl-acetate (EtAct) and insoluble in n-hexane. The solubility of these insecticides in aqueous solution and the partitioning of these insecticides from aqueous solution into EtAct are independent of the pH of the aqueous solution. At pH 8, Me did not react with o-phthalaldehyde (a NH2 selective dye), but gamma-amino-butyric acid (pKa 10) did. Thus, despite the presence of an amino group, Ac and Me do not exhibit pH dependent solubility in aqueous and in organic solvents. Ac has two O atoms with non-bonding electrons (P = O delta- and C = O delta-) where P = O and C = O point in opposite directions. Me has only one O atom with non-bonding electrons (P = O delta-). However, because of charge translocation, the C = O group of Ac exists as C = O- and the P-NH3+ group of Me exists as P = NH2+ at a pH lower than their pKa. The P-N bond of Me, but not of Ac, is hydrolyzed at pH 2. Thus, the presence of an electron rich domain stabilizes Ac's P-N bond. The CH3S-P bond of both insecticides is similarly hydrolyzed at pH 11. This indicates that the two compounds are considerably similar except that Ac has an additional electron rich domain. At physiological pH, therefore, the functional differences between these insecticides may be due to the differences in their electronic structure. We propose that, similar to a previous model for cationic inhibitors of AChE (13), the P = O delta- group of Me forms hydrogen bonds within the oxyanion-hole causing the leaving group (-SCH3) to orient towards the "gorge" opening. This orientation allows the P atom of Me to interact with Ser200, resulting in the phosphorylation of the enzyme. For acephate, either P = O or C = O, but not both, interact within the oxyanion-hole. This destabilizes the binding of Ac to the active center, resulting in reduced AChE phosphorylation.