Tertiary N-haloethylamines are able to cyclize to the corresponding aziridinium ions. The inhibitory activity of the DSP-4 (N-(o-brombenzyl)-N-ethyl-2-chlorethylamine) and its two derivatives OS-21 (N-benzyl-N-ethyl-2-chloroethylamine) and OS-23 (N-fenylethyl-N-ethyl-2-chloroethylamine) was studied toward rat brain acetylcholinesterase (AChE) in vitro. The influence of the THA (tacrine; 9-amino-1,2,3,4-tetrahydroacridine) on AChE inhibition by these substances was also evaluated. The results demonstrated that all of three aziridinium compounds formed in solution caused a time- and concentration-dependent irreversible enzyme inhibition. The association of aziridinium compounds with the AChE was a relatively slow second-order reaction. DSP-4 showed the fastest rate of AChE alkylation, OS-21 had a lowered rate and OS-23 displayed the lowest rate. Pretreatment of the enzyme by THA decreased the rate of alkylation by all three aziridinium compounds by allosteric mechanism.