Pituitary corticotropic cells express a specific vasopressin receptor, called V1b or V3, through which vasopressin stimulates corticotropin secretion. We recently cloned a cDNA coding for this receptor and showed that it belongs to the G protein-coupled receptor family. V3 mRNA is readily detected by RT-PCR in normal human pituitaries and corticotropic pituitary adenomas but not in PRL or GH-secreting adenomas, thus demonstrating that, like POMC itself and the CRH receptor, V3 is a marker of the corticotropic phenotype. Nuclease protection experiments suggest that V3 is overexpressed in some corticotropic adenomas, and thus may play a role in tumor development by activating the phospholipase C-signalling pathway. In addition analysis of its expression in nonpituitary neuroendocrine tumors showed a striking association with carcinoids of the lung responsible for the ectopic ACTH syndrome.