We examined the inhibitory activity of (1,3-dioxolan-2-yl and 1,3-dioxan-2-yl)methylaminium derivatives(A; 1-8) against acetylcholinesterase. Derivatives of six-membered 1,3-dioxane exhibited more potent inhibitory effect than corresponding 5-membered 1,3-dioxolanes. The presence of methyl group at C4 position of dioxane ring was effective to increase the inhibitory potency of heterocyclic analogues. The activity of N-phenacyl-aminiums was greater than that of N,N,N-trimethyl-aminiums. In general, the terminal methyl group on 1,3-dioxane ring and the phenacyl group in ammonium compound A were assumed to be important factors to enhance the inhibitory action.