Starting from the 2.8-Å resolution X-ray structure of bovine rhodopsin, three-dimensional molecular models of the complexes between arginine-vasopressin (AVP) and two receptor subtypes (V_1a, V_1b) have been built. Amino acid sequence alignment and docking studies suggest that four key residues (1.35, 2.65, 4.61, and 5.35) fine tune the binding of vasopressin and related peptide agonists to both receptor subtypes. To validate these predictions, a series of single or double mutants were engineered at V_1a and V_1b receptor subtypes and tested for their binding and functional properties. Two negatively-charged amino acids at positions 1.35 and 2.65 are key anchoring residues to the Arg8 residue of AVP. Moreover, two amino acids (V^4.61 and P^5.35) delineating a hydrophobic subsite at the human V_1b receptor are responsible for the recognition of V_1b selective peptide agonists. Last, one of the latter positions (5.35) is hypothesized to explain the pharmacological species differences between rat and human vasopressin receptors for a V_1b peptide agonist. Altogether these refined 3-D models of V_1a and V_1b human receptors should enable the identification of further new selective V_1a and V_1b agonists as pharmacological but also therapeutical tools.