Acephate pre-exposure provided protection against the inhibition of RBC and brain acetylcholinesterase (AChE), and plasma cholinesterase (ChE) activities in rats exposed to both acephate and methamidophos. In vitro addition of acephate to AChE prior to or with methamidophos also provided complete protection against AChE inhibition by methamidophos. When acephate was added to the enzyme after methamidophos, its protective effect decreased with increasing time between the additions. Since acephate has greater affinity than does methamidophos for the AChE active site (Singh, A.K., Toxicol. Appl. Pharmacol., 81 (1985) 302), it is proposed that acephate provided protection by binding with the AChE active site and, therefore, preventing methamidophos from binding with the enzyme. It is also proposed that acephate prevented the initial competitive binding of methamidophos to the AChE active site and delayed the initial sequence of events essential for phosphorylation of AChE.