The interaction of gallamine with muscarinic receptors from different tissues has been investigated. Gallamine binds to a site distinct from the conventional muscarinic ligand binding site and modulates the binding of agonists and antagonists to the conventional binding site. In agreement with reported pharmacological studies, the effects of gallamine on the binding of muscarinic ligands are much greater in heart than in other tissues. These findings suggest the possibility of developing novel and selective muscarinic drugs.