The V2 vasopressin renal receptor (V2R), which controls antidiuresis in mammals, is a member of the large family of heptahelical transmembrane (7TM) G protein-coupled receptors (GPCRs). Using the automated GPCR modeling facility available via Internet (http:/(/)expasy.hcuge.ch/swissmod/SWISS-MODEL.+ ++html) for construction of the 7TM domain in accord with the bovine rhodopsin (RD) footprint, and the SYBYL software for addition of the intra- and extracellular domains, the human V2R was modeled. The structure was further refined and its conformational variability tested by the use of a version of the Constrained Simulated Annealing (CSA) protocol developed in this laboratory. An inspection of the resulting structure reveals that the V2R (likewise any GPCR modeled this way) is much thicker and accordingly forms a more spacious TM cavity than most of the hitherto modeled GPCR constructs do, typically based on the structure of bacteriorhodopsin (BRD). Moreover, in this model the 7TM helices are arranged differently than they are in any BRD-based model. Thus, the topology and geometry of the TM cavity, potentially capable of receiving ligands, is in this model quite different than it is in the earlier models. In the subsequent step, two ligands, the native [arginine8]vasopressin (AVP) and the selective agonist [D-arginine8]vasopressin (DAVP) were inserted, each in two topologically non-equivalent ways, into the TM cavity and the resulting structures were equilibrated and their conformational variabilities tested using CSA as above. The best docking was selected and justified upon consideration of ligand-receptor interactions and structure-activity data. Finally, the amino acid residues were indicated, mainly in TM helices 3-7, as potentially important in both AVP and DAVP docking. Among those Cys112, Val115-Lys116, Gln119, Met123 in helix 3; Glu174 in helix 4; Val206, Ala210, Val213-Phe214 in helix 5; Trp284, Phe287-Phe288, Gln291 in helix 6; and Phe307, Leu310, Ala314 and Asn317 in helix 7 appeared to be the most important ones. Many of these residues are invariant for either the GPCR superfamily or the neurophyseal (vasopressin V2R, V1aR and V1bR and oxytocin OR) subfamily of receptors. Moreover, some of the equivalent residues in V1aR have already been found critical for the ligand affinity.