2. On the second admission one week later, the procedures will be identical to the first admission with following exceptions: (a) instead of tolvaptan, at 0900 hours, subjects will be started on an infusion of 3% saline at a rate of 0.033 mL/min/kg body weight. This infusion will continue for 6 hours (i.e. until 1500 hours) and is expected to raise plasma sodium about 7 millimoles/L), the same as the tolvaptan treatment;(b) the volume of blood removed each time will be smaller (10 mL) since they will not be assayed for tolvaptan; (c)subjects will not be permitted to drink during the infusion. In addition, subjects will remain in the unit for only 4 more hours (until 1900 hours). During this time, they will be permitted to drink at will, will be served dinner at about 1700 hours and will have all baseline measurements repeated at 1700 and 1900 hours.