Arginine vasopressin (AVP) modulates the secretion of ACTH through vasopressin receptor subtype V1b in the pituitary. We recently cloned human V1b, but several inconsistencies were found between the characteristics of this cloned receptor and those of rat pituitary membrane shown by previous workers. To clarify this issue, we report here the molecular cloning and functional expression of the cDNA encoding V1b in rat pituitary. This receptor encodes 425 amino acid protein having the highest identity with the human V1b (81%). Expression of this receptor in COS-1 cells showed pharmacological characteristics of V1b consistent with those of rat pituitary membrane. Northern blot and RT-PCR analyses revealed that mRNA of this receptor was expressed not only in anterior pituitary, but also in other tissues. This finding raises the possibility that V1b may play a role in regulating cell functions of these tissues.