Research on muscarinic receptors is advancing at an extraordinary rate. Ten years ago, the existence of muscarinic receptor sub types was a logical assumption with only scattered experimen tal support. The discovery that pirenzepine recognized apparent heterogeneity in muscarinic binding sites infused new life into the problem of subclassifying muscarinic receptors. Simultaneous advances in molecular biology created a frenzy to clone cell sur face receptors. The muscarinic receptor succumbed surprisingly quickly, revealing its structure and that of at least four closely related gene products...