It has been observed repeatedly^1,2 that the inhibitor of cholinesterases tacrine (1,2,3,4-tetrahydroacridin-9-amine) inhibits the binding of orthosteric muscarinic ligands to muscarinic receptors with unusually steep binding curves (high Hill slopes about value 1.5). Definite explanation of this phenomenon is not available. A mechanistical model has been proposed according to which tacrine binds to the orthosteric site and to the allosteric site simultaneously with homotropic positive cooperativity ³. At the same time, the binding of tacrine to the allosteric site has a negative efect on the binding of the commonly used radiolabelled muscarinic ligands ([³H]N-methylscopolamine, or [³H]quinuclidinyl benzilate) to the classical site. Another hypothesis suggests that tacrine binds to two spatially separated allosteric sites on muscarinic receptor with positive cooperativity or its binding to the common allosteric site modulates receptor-receptor interactions ⁴. We have discovered recently that structurally closely related compounds 7-methoxytacrine, acridin-9-amine, and proflavine (acridine-3,6-amine) also yield characteristically steep binding curves while quinacrine (6-chloro-9-([4-diethylamino]-1-methylbutyl)amino-2-methoxyacridine) behaves like a competitor and displaces the orthosteric ligand [³H]N-methylscopolamine yielding the standard curves with Hill slope close to 1. These experimental findings are rather surprising. First, tacrine has been synthesized as a three-dimensionally voluminous analogue of acridin-9-amine in order to demonstrate that the complete planar acridine nucleus is required for antibacterial activity. Nevertheless, their effects at muscarinic receptors are almost identical. On the other side, allosteric modulators proflavine and acridin-9-amine, together with ‘orthosteric’ competitor quinacrine belong to the same category of compounds which possess the very special mechanisms of action: intercalation, stacking and self-aggregation ⁵. We hypothesize that the bulky substitution at position 9 on basic acridine skeleton sterically hinders the simultaneous binding quinacrine into orthosteric and allosteric binding sites while the molecules of tacrine, or 7-methoxytacrine, respectively, can stack themselves, as do molecules of proflavine, or acridin-9-amine, inside the receptor pocket.