Molecular modifications have been made on a highly potent, active antagonist to organophosphate-induced toxicity, 4,4'-bis[1,3-dioxan-2-ylmethyl)methylamino]acetyl]biphenyl dimethobromide (1). Stepwise removal of the oxygen atoms from the dioxane rings, as well as changing the position of attachment of substituents on the 1,3-dioxane rings and decreasing the ring size from six-membered to five-membered caused drastic or complete loss of pharmacological effect. Partial structures of 1 were all inactive. Thus, the structure of 1 seems to be remarkably specific. Additional pharmacological data are reported for 1.