We have reported that [Arg⁸]-vasopressin-stimulated insulin release is blunted in islet cells isolated from V1b receptor-deficient (V1bR-/-) mice. In this study, we used V1bR-/- mice to examine the physiological role of the V1b receptor in regulating blood glucose levels in vivo, and we found that the fasting plasma glucose, insulin, and glucagon levels were lower in V1bR-/- mice than in wild-type (V1bR+/+) mice. Next, we evaluated glucose tolerance by performing an intraperitoneal glucose tolerance test (GTT). The plasma glucose and insulin levels during the GTT were lower in V1bR-/- mice than in V1bR+/+ mice. An insulin tolerance test (ITT) revealed that, after insulin administration, plasma glucose levels were lower in V1bR-/- mice than in V1bR+/+ mice. In addition, a hyperinsulinemic-euglycemic clamp study showed that the glucose infusion rate was increased in V1bR-/- mice, indicating that insulin sensitivity was enhanced at the in vivo level in V1bR-/- mice. Furthermore, we found that the V1b receptor was expressed in white adipose tissue and that insulin-stimulated Akt phosphorylation was increased in adipocytes isolated from V1bR-/- mice. Thus, the blockade of the V1b receptor could result, at least in part, in enhanced insulin sensitivity by altering insulin signaling in adipocytes.