1. To test the hypothesis that the variable effect of tolvaptan on urine concentration and flow depends on the level of plasma vasopressin as well as plasma tolvaptan, the relationship between the osmolarity of the first 9 urine collections after treatment to the area under the curve (AUC) for plasma vasopressin, for plasma tolvaptan and for the ratio of plasma tolvaptan/plasma vasopressin during the concurrent time periods will be determined by regression analysis in each of the 12 subjects. The same analysis will be performed using urine volume instead of urine osmolarity. Hypothesis 1 predicts that in all subjects, the best correlation (r value)will be between urine osmolarity and the ratio of tolvaptan/vasopressin. It also predicts that the slopes of every individual regression line will be negative (i.e. the higher the ratio of tolvaptan to vasopressin)the lower the urine osmolarity). However, the slopes may differ between subjects owing to individual differences in the concentrating capacity of the kidneys.