The optical isomers of the N-methyl-D-aspartate (NMDA) receptor ion-channel blocker dizocilpine (MK-801) were shown to interact with electric eel and rat brain acetylcholinesterase (AChE) in a mixed competitive-noncompetitive way. The (-) form, pharmacologically less active, was the most potent of the two isomers as an AChE inhibitor (Ki for electric eel and rat brain AChE being 6.2 and 17.9 microM, respectively, compared with 200 and 450 microM, respectively, of the (+) form). Both enantiomers premixed with AChE preparations, dose-dependently protected the enzyme from inactivation by diisopropylfluorophosphate (DFP). The maximal protective effects against 40 and 10 microM DFP were in the ranges 10.7-23.8 and 19.5-31.4% of control enzymic activity for the (+) and (-) forms of dizocilpine, respectively. The extent of the protective effect against DFP was increased up to 80.1% of control enzymic activity for (-)-dizocilpine and to 38.4% for (+)-dizocilpine by diluting the enzymic mixtures 1000 times after treatment with the organophosphate agent. The two enantiomers added to AChE 15 min after DFP, failed to reactivate the enzyme. Finally, it was shown that (+)- and (-)-dizocilpine dose-dependently and competitively decreased the DFP bimolecular reaction constant, K(i). We conclude that dizocilpine exerts a protective action towards AChE against irreversible DFP inhibition, but the molecular mechanism of such an action is at present unclear.