Study Objective: To understand the response of mice lacking the insulin-regulated aminopeptidase (IRAP) to an acute water load. Hypothesis: For mammals to respond appropriately to acute water loading, vasopressin activity needs to decrease. IRAP degrades vasopressin in vivo. Therefore, we hypothesized that mice lacking IRAP have impaired ability to degrade vasopressin and thus have persistent urinary concentration. Methods: Age matched 8 to 12-week-old IRAP wild-type WT and KO male mice were used for all experiments. Blood electrolytes and urine osmolality were measured before and one hour after water load (~2ml sterile water via intraperitoneal injection). Urine was collected from IRAP WT and KO mice for urine osmolality measurements at baseline and after 1-hr administration of the vasopressin type 2 receptor antagonist, OPC 31260 (10mg/kg/IP). Immunofluorescence and immunoblotting were performed on kidneys at baseline and after 1-hr acute water load. Results: IRAP was expressed in the glomerulus, thick ascending loop of Henle, distal tubule, connecting and collecting ducts. IRAP KO mice have elevated urine osmolality when compared to WT mice due to higher membrane expression of aquaporin 2 (AQP2), which is restored to that of controls after administration OPC 31260. IRAP KO mice developed hyponatremia after an acute water load, because they were unable to increase free water excretion due to increased surface expression of AQP2. Conclusions: IRAP is required to increase water excretion in response to an acute water load due to persistent vasopressin stimulation of AQP2