Various compounds are known to allosterically modulate the binding of ligands to muscarinic receptors. Most of these compounds have another predominant pharmacological action. Identification of the potent representatives should be useful for the development of allosteric modulators that are specific and highly active. For various reasons, a direct comparison of allosteric potencies on the basis of literature data is difficult. Therefore, a series of compounds was compared with regard to the allosteric delay of the dissociation of N-[3H]methylscopolamine from porcine heart M2 receptors under the following assay conditions: "Na,K,Pi buffer", 4 mM Na2HPO4, 1 mM KH2PO4, pH 7.4, 23 degrees C; "Mg,Tris,Cl,Pi buffer', 50 mM Tris-HCl, 3 mM MgHPO4,pH 7.3, 37 degrees C. Generally, the allosteric potency of the compounds was higher in the Na,K,Pi buffer, compared with the Mg,Tris,Cl,Pi buffer. However, the extent of the potency shift differed, ranging from approximately 2-fold for tacrine to approximately 100-fold for gallamine. The concentration retarding radioligand dissociation to half of the control rate (EC50) served as a measure of allosteric potency. Under both assay conditions, alcuronium was the most potent compound (EC50,Na,K,Pi = 4 nM and EC50,Mg,Tris,Cl,Pi = 55 nM), followed by alkane-bisammonium and bispyridinium compounds containing phthalimido moieties. Gallamine showed intermediate potency (EC50 values of 180 nM and 16,000 nM in Na,K,Pi buffer and Mg,Tris,Cl,Pi buffer, respectively). Obidoxime and hexamethonium, both known to antagonize allosteric actions, revealed submaximal efficacy and low potency (EC50,Na,K,Pi of approximately 100,000 nM). The relevance of these results, regarding the identification of lead structures for the development of new allosteric modulators, is discussed.