This article discusses the structural and functional features of a new family of membrane receptors including alpha-adrenergic and beta-adrenergic receptors for catecholamines, muscarinic receptors for acetylcholine, and receptors for histamine, dopamine, serotonin, and neuropeptides such as angiotensin. All these receptors are coupled to GTP-binding proteins, called G proteins. All G proteins have similar membrane topologies with a single peptide chain composed of seven membrane-spanning domains separating a series of successive extracellular and intracellular portions. Results of studies of beta-adrenergic receptors suggest that some amino acids in the seven membrane-spanning domains are part of the ligand-binding site, whereas the protein G-coupling site seems to involve amino acids located in the third cytoplasmic loop and in the C-terminal extremity, which also contains the phosphorylation sites. Genes encoding this family of receptors exhibit sequence homologies and also contain sequences which may be involved in the regulation of the level of expression of these receptors. Recent development of a receptor-expressing bacterial system can be expected to provide significant advances in the accurate investigation of receptor structure-function correlations.