The present study aims at delineating residues in the vasopressin/oxytocin receptor family responsible for the high affinity binding of the hormone. Therefore, we have constructed a computer-generated 3 dimensional model of the rat V1a vasopressin receptor subtype which allowed us to propose residues likely to be involved in agonist binding. Among these residues, several are highly conserved in the receptor family. They were selected for site-directed mutagenesis on the basis of putative direct interaction with bound ligands. The present model and experimental results led us to conclude that the hormone is docked in a pocket completely buried in the transmembrane core of the receptor. Large polar residues, such as glutamine and lysine, located in transmembrane regions 2,3,4 and 6 are involved in the binding of the neurohypophysial hormone. Since all the mutated residues are highly conserved in AVP and OT receptors, we propose that the agonist binding site is similar in all members of the receptor family; only minor changes were found in antagonist potencies, suggesting that agonist and antagonist binding sites do not completely overlap.