The effects of tolvaptan on serum sodium (mean ± SE) in patients with and without hyponatremia (n = 21 and n = 18, respectively). Tolvaptan (15 mg/d, 5-14 d) treatment (black column) significantly increased serum sodium concentration [t (40) = -4.029, bP < 0.01 vs before treatment group] in patients with hyponatremia, but not in patients without hyponatremia [t (32) = 1.545, P > 0.05 (NS)]. NS: Not significant.