The ternary allosteric model predicts the possibility of discovering molecules with novel and highly subtype-selective modes of action. This approach has been applied to muscarinic receptors. The alkaloid brucine is capable of selectively enhancing by an allosteric mechanism the effects of low but not high concentrations of acetylcholine at only the m1 subtype of muscarinic receptors. A simple derivative of brucine, N-chloromethylbrucine, enhances acetylcholine actions selectively at only m3 receptors. In addition it binds to, but does not affect, the properties of m4 receptors, thereby demonstrating neutral cooperativity and an 'absolute' selectivity of action at m3 receptors over m4 receptors. Brucine N-oxide enhances acetylcholine binding at m3 and m4 receptors and is neutral at m1 and m5 receptors. These findings allow the possibility of developing muscarinic agents that have a novel and highly targeted mode of action; they may act only on a single muscarinic receptor subtype which is functioning sub-optimally and therefore be of use therapeutically in the early stages of Alzheimer's Disease.