In 1914, Sir Henry Dale provided the basis for the classical and comfortable definition of muscarinic and nicotinic acetylcholine receptors: Muscarinic receptors are selectively activated by muscarine and blocked by atropine; nicotinic receptors are activated by nicotine and blocked by curare. This definition lasted over 60 years, despite isolated reports that the picture might not be so simple. We now know, as the result of molecular biological studies, that there are multiple variants of both muscarinic and nicotinic receptors. These receptors are members of two quite separate gene superfamilies and only share the property of being activated by the same ligand, acetylcholine. The aim of this review is to discuss the structure, function, and binding properties of the different muscarinic receptor species, attempting where possible to coordinate the diverse experimental data into a uniform picture. For a more comprehensive review on muscarinic receptors, readers are di rected to a recent book by Brown (l).