Oxytocin (OT) is one of the secretagogues for stress-induced adrenocorticotropoic hormone (ACTH) release. OT-induced ACTH release is reported to be mediated by the vasopressin V1b receptor in the rat pituitary gland, which contains both OT and V1b receptors. We examined OT-induced ACTH release using primary cultures of anterior pituitary cells from wild-type (V1bR^+/+) and V1b receptor knockout (V1bR^-/-) mice. OT stimulated similar levels of ACTH release from pituitary cells of V1bR^+/+ and V1bR^-/- mice. OT-induced ACTH release was significantly inhibited by the selective V1b receptor antagonist SSR149415 and by the OT receptor antagonist CL-14–26 in V1bR^+/+ mice. In addition, co-treatment with SSR149415 at 10^-6 M and CL-14–26 at 10^-6 M inhibited OT-induced ACTH release to the control level in V1bR^+/+ mice. In V1bR^-/- mice, OT-induced ACTH release was significantly inhibited by CL-14–26 at 10^-8 M and completely inhibited at 10^-7 M. These results indicate that OT induces the ACTH response via OT and V1b receptors in V1bR^+/+ mice but via only OT receptors in V1bR^-/- mice. The gene expression level of the OT receptor was significantly higher in the anterior pituitary gland of V1bR^-/- mice than in that of V1bR^+/+ mice, suggesting that the OT receptor is up-regulated to compensate for ACTH release under conditions of V1b receptor deficiency.