3. To test the hypothesis that the variable effect of tolvaptan on plasma sodium can be reduced by continuously adjusting fluid intake to a level below urine output by a constant amount (5 mL/kg each hour), the relation of plasma sodium to elapsed time over the 6 hour period after tolvaptan and during saline infusion will be calculated separately for each subject by regression analysis and compared. If the hypothesis is correct, the correlation coefficients for the tolvaptan and saline treatments should be uniformly high e.g. greater than 0.9 in all subject even though the slopes of the lines describing these relationships may differ.