T he structural organization and 5'—flanking region of the mouse Vla and Vlb vasopressin receptor genes were determined. The mouse Vla receptor gene was located within an 8—kb XbaI fragment, and the mouse Vlb receptor gene was located within a l4-kb EcoR Vfragment. Both genes were comprised of two coding exons that were separated by a 2.3—kb and a 8.0-kb intron, respectively, located before the respective seventh transmembrane domain of the receptor sequence. The availability of these genes would allow us to study the functional role of Vla and Vlb receptors by disrupting the gene in mice.