Several studies demonstrated that pretreatment with reversible acetylcholinesterase (AChE) inhibitor, such as (pyridostigmine) PYR, improved the survival of animals intoxicated by organophosphate nerve agents (OP). These compounds temporarily inhibited a fraction of the enzyme and protected it from inactivation by nerve agents. An important criterion for effective pretreatment is that it must ensure the recovery of the protected fraction of the enzyme. We thus designed a simple ex vivo method to investigate the recovery of AChE activity, that was protected by PYR, prior to irreversible inhibition by an OP, using a modified Ellman assay. Results show that our approach is suitable for routine use and can successfully predict the potential use of various AChE inhibitors as pretreatment drugs against OP nerve agent intoxication.