The cloning of cDNAs and genes for five different muscarinic acetylcholine receptors provides a new basis for characterizing muscarinic receptor function. Studies of the cloned receptors when introduced into cells not expressing endogenous receptors have allowed the initial identification of two classes of functional response. The m1, m3 and m5 receptors belong to a class characterized by agonist-induced stimulation of phosphatidylinositol metabolism and are structurally more related to each other than they are to the m2 and m4 receptors, which belong to a class associated with agonist-induced inhibition of adenylate cyclase. While functional differences within these classes may yet be found, it appears likely that much of the difference between functionally similar receptors will be found to lie in their regulation.