*Conivaptan is an arginine vasopressin V1A and V2 receptor antagonist. The intravenous formulation is approved in the US for use in the treatment of euvolemic and hypervolemic hyponatremia. Conivaptan produces a dose-dependent electrolyte-sparing aquaresis (solute-free water excretion), increasing serum sodium levels. *In a randomized, double-blind, parallel-group, placebo-controlled, multicenter trial in adults with euvolemic or hypervolemic hyponatremia, the area under the serum sodium concentration-time curve over a 4-day treatment duration (primary endpoint) was significantly greater in intravenous conivaptan 40 mg/day recipients than in placebo recipients. *The total time during treatment that patients had serum sodium levels > or = 4 mEq/L above baseline was significantly longer in intravenous conivaptan than placebo recipients. In conivaptan recipients, an increase in serum sodium levels of > or = 4 mEq/L above baseline was achieved approximately 1 day after the first dose of the drug. *In addition, the mean change from baseline in free water clearance and effective water clearance over the first day of treatment was significantly greater with intravenous conivaptan than with placebo. *Given the nature of the treatment, the tolerability profile for intravenous conivaptan was generally acceptable in patients with hyponatremia. The most common adverse events were injection related (e.g. injection-site phlebitis), hypotension, and pyrexia.