2. To test the hypothesis that the individual differences in plasma vasopressin during treatment are due to the inherent,genetically determined differences in the secretory response to osmotic (sodium) stimuli that are known to exist in humans, the relationship of plasma vasopressin to plasma sodium (or osmolarity)during the first 6 hours of treatment with tolvaptan or 3% saline will be determined separately by regression analysis and compared. Based on previous findings, the correlation of plasma vasopressin to plasma sodium (or osmolarity) should be very high (r>0.9) in every subject during 3% saline infusion but the slopes will vary markedly between subjects. The hypothesis predicts that these correlations and marked differences in slopes will not differ after tolvaptan treatment. This prediction will be tested by regressing the slope during tolvaptan against the slope in the same subject during 3% saline infusion in all 12 subjects. If the hypothesis is correct, the correlation coefficient between the individual slopes will be greater than 0.8 and the line describing the relation will have a slope approximating 1 with an intercept near zero on both axes.