The recently-determined structure of rhodopsin has provided a suitable basis for modeling the three-dimensional structure of the M1 muscarinic acetylcholine receptor. Using this as a framework for interpreting mutagenesis studies, we have been able to suggest most of the contacts which the receptor makes with acetylcholine and many of the intramolecular contacts which are important for the ground-state structure of the receptor. It is possible to outline a mechanism of G-protein interaction.