Organophosphorus pesticides such as parathion or chlorpyrifos are substances used worldwide foragricultural purposes. These compounds are able to inhibit an enzyme acetylcholinesterase (AChE; EC188.8.131.52) by phosphorylation in its active site. AChE reactivators and anticholinergics are generally used asantidotes in the case of intoxication by these agents. In this work, the reactivation potency of ninestructurally different AChE reactivators was tested in vitro. Chlorpyrifos was chosen as an appropriatemember of the pesticide family. The result is that bisquaternary reactivators with two oxime groups inposition four at the pyridinium rings (trimedoxime and K074) seem to be the most potent reactivators ofchlorpyrifos-inhibited AChE.