1. The molecular composition of acetylcholinesterase (AChE) obtained from cockroach neural, and rat brain tissues was different. Vertebrate enzyme exhibited a higher degree of polymerization than insect enzyme. 2. Acephate was a potent inhibitor of cockroach AChE, but a poor inhibitor of rat AChE. Unlike acephate, methamidophos was a potent inhibitor of both cockroach and rat enzymes. Acephate exhibited greater affinity for the cockroach-AChE than for the rat-AChE, and acephate phosphorylated the cockroach-AChE several times faster than the rat enzyme. The rate of phosphorylation of insect and rat AChE was similar in the presence of methamidophos. Solubilization of AChE by Triton X-100 altered the kinetics of inhibition of rat AChE by acephate. However, solubilization did not alter the kinetics of inhibition of rat AChE by methamidophos or the kinetics of inhibition of cockroach AChE by acephate or methamidophos. 3. The mechanism of acephate-cockroach AChE interaction was different than the mechanism of acephate-rat AChE interaction. It is proposed that both the rat and cockroach enzyme may contain, along with the anionic and esteratic sites, an "electron deficient" (ED) binding site which may exhibit selectivity for acephate and nefopam. The ED site in rat-AChE has allosteric properties, whereas the cockroach-AChE does not. It is also proposed that the ED site in cockroach-AChE may be situated in or adjacent to the active site and, therefore, acephate may be bound to the ED site such that the phosphate moiety of acephate interacts with the enzyme's "esteratic" site. Although nefopam also bound to the ED site in cockroach AChE, it did not inhibit the enzyme. This study also indicated that the ED site in rat-AChE may be peripheral to the active site, and that the binding of acephate to this site prevented the phosphorylation by methamidophos of the rat-AChE. Unlike acephate, methamidophos specifically bound to the active site in both the rat- and cockroach-AChE.