In this investigation of two insecticides, methamidophos was at least 75 to 100 times more potent an inhibitor of acetylcholinesterase (AChE) and cholinesterase (ChE) than was acephate. For brain samples, the bimolecular rate constant values (Ki) in the presence of acephate and methamidophos were 0.06 and 6.3 μm-1 · min-1, respectively. The affinity (as measured by Ka values) of acephate was greater than that of methamidophos to both the AChE and the ChE activesites. The difference in potencies of the two insecticides was entirely the result of a difference in the rate of their phosphorylation (Kp) of AChE. The Kp values in the presence of acephate and methamidophos for brain samples were 14.3 and 0.13 min-1, respectively. The inhibition potency [as measured by Ki and the concentration required to cause 50% inhibition in AChE or ChE activity (IC50 values)] of methamidophos, but not of acephate, increased by lengthening the enzyme-inhibitor incubation time. This increase in potency was the result of an increase in Kp. The results indicate that brain AChE was relatively more sensitive than red blood cell (RBC) AChE to this insecticide. Variations in the sensitivities of brain and RBC AChE activities to these insecticides are because of differences in the affinity of insecticides to the enzyme's active site. The Ki and IC50 values for plasma ChE were altered by changing the substrate. The differences in the sensitivities of acetylthiocholine (ATCh), butyrylthiocholine (BTCh), and propionylthiocholine (PTCh) hydrolyzing activities of plasma to exposure to methamidophos were because of differences in Ka and Kp values. However, differences in the sensitivities of BTCh or PTCh hydrolyzing activity of plasma to exposure to acephate cannot be attributed entirely to differences in Ka and Kp values.