1. The treatment on the first admission will be tolvaptan. Six subjects will receive tolvaptan at a dose of 60 mg by mouth and six subjects will receive tolvaptan at 30 mg by mouth. Hourly for the next 6 hours (i.e. until 1500 hours), all the measurements and samples obtained at baseline will be repeated as before. In addition, the subject will drink a volume of water that is 5 mL/kg body weight lower than the volume of urine just voided. This is expected to raise plasma sodium by about 7 millimoles/L in 6 hours. For the next 16 hours, subjects will remain and be allowed to drink at will. In addition, they will be served dinner at approximately 1700 hours and all the measures obtained at baseline will be repeated again at 1700, 1900, 2300 and 0700 hours when subjects will be discharged.