Despite the numerous studies that have been spawned by the cloning of more than 240 G-protein-coupled receptors, the molecular basis for receptor discrimination of receptor-ligand interactions remains a central issue in membrane receptor biology. The receptor's criteria for agonists and antagonists allow these types of ligands to compete for the same binding site on the receptor, but only agonists are able to stimulate intracellular signaling. Various vasopressin agonists and antagonists, which are known to have different binding affinities for the V1a and V2 vasopressin receptors, can be exploited in the search for the conformational changes that precede and accompany receptor activation. Because the V1a and V2 vasopressin receptors are coupled to different intracellular signaling systems, it should be possible to assay the functional components of binding and G-protein coupling in a series of chimeric receptors. With the ever-increasing database on the structural determinants of G-protein-coupled receptor function, at least some of the underlying mechanisms of transmembrane signal transduction should be better understood in the next few years.