The study of antagonist-binding domains of the human V1a vasopressin receptor was performed using a radioiodinated photoreactive peptide antagonist. This ligand displayed a high affinity for the receptor expressed in Chinese hamster ovary cell membranes, and specifically labeled two protein bands with apparent molecular mass at 85-90 and 46 kDa. Our results clearly show that the V1a receptor is degraded during incubation with the ligand and that the 46-kDa species is probably the result of the 85-90-kDa species proteolytic cleavage. Truncation of the receptor was then confirmed by deglycosylation with N-glycosidase F. A monoclonal antibody directed against a c-Myc epitope added at the receptor NH2 terminus allowed immunoprecipitation of the 85-90-kDa photolabeled species. The 46-kDa photolabeled protein never immunoprecipitated, indicating that the truncated form of the receptor lacks the NH2 terminus region. To localize photolabeled domains of the receptor, the 46-kDa protein was cleaved with V8 and/or Lys-C endoproteinases. The identity of the smallest photolabeled fragment, observed at approximately 6 kDa, was then confirmed by mutation of the potential V8 cleavage sites. Our results indicate that covalent labeling of the vasopressin V1a receptor with the photoreactive antagonist occurs in a region including transmembrane domain VII (residues Asn327-Lys370).