Recent epidemiological studies have revealed novel relationships between low water intake or high vasopressin (AVP) and the risk of hyperglycemia and diabetes. AVP V_1A and V_1B receptors (R) are expressed in liver and pancreatic islets, respectively. The present study was designed to determine the impact of different levels of circulating AVP on glucose homeostasis in normal Sprague Dawley rats, and the respective roles of V_1AR and V_1BR. We showed that acute injection of AVP induces a dose-dependent increase in glycemia. Pretreatment with a selective V_1AR antagonist, but not a V_1BR antagonist prevented dose-dependently the rise in glycemia. V_1BR antagonism did not modify the hyperinsulinemic response resulting from AVP-induced hyperglycemia, but enhanced the fall in glucagonemia. Acute administration of selective V_1AR or V_1BR agonists confirmed the involvement of V_1AR in the hyperglycemic effect of AVP. In chronic experiments, AVP levels were altered in both directions. Sustained AVP infusion through implantable minipumps induced a time-dependent increase in fasting glycemia, whereas lowering endogenous AVP by increasing water intake had no effect. After 4 weeks of AVP infusion, the rise in glycemia amounted to 1.1 mmol/L (p<0.01) without significant change in insulinemia. This effet was attenuated by cotreatment with a V_1AR antagonist. Similar results were observed in lean Zucker rats. These findings demonstrate for the first time a causal link between chronic high AVP and hyperglycemia through V_1AR activation and thus provide a pathophysiological explanation for the relationship observed in human cohorts between the AVP-hydration axis and the risk of diabetes.