The bis(dichlorobenzyl) ether of the bispyridinium oxime TMB 4 stabilizes antagonist binding to M2-cholinoceptors which is indicative of an allosteric action. More than 10 derivatives of the lead compound were synthesized to investigate structure-activity relationships. The allosteric potency of the compounds was indicated by the concentrations which retarded the rate of dissociation of [3H]N-methylscopolamine from porcine cardiac cholinoceptors by a factor of 2 (EC50). Compared with TMB 4, the bis(dichlorobenzyl) derivative 4a displayed a more than 200-fold higher potency (EC50 = 4.7 microM). One of the dichlorobenzyl groups could be replaced by a methyl group without loss of activity (EC50 = 4.5 microM). Further shortening of this end of the molecule was accompanied by a moderate decline in potency to a minimum of EC50 = 26 microM. The second quaternary nitrogen was not a prerequisite for an allosteric activity. It is concluded that one half of the lead compound is pivotal for an interaction with the allosteric site of the M2-cholinoceptor, whereas the opposite end of the molecule modulates the allosteric activity.