Objective and design: Most cases of euvolaemic hyponatraemia are associated with elevated plasma levels of AVP. Conivaptan is a high-affinity, nonpeptide vasopressin V1A/V2-receptor antagonist. We performed a subgroup analysis of a multicentre, randomized, placebo-controlled, double-blind, parallel-group study to evaluate the efficacy and safety of intravenous (i.v.) conivaptan for the treatment of euvolaemic hyponatraemia. Patients: Fifty-six euvolaemic patients with serum [Na+] of 115 to < 130 mmol/l received conivaptan 40 or 80 mg/day or placebo via continuous i.v. infusion for 4 days. A 20-mg loading dose was administered intravenously over 30 min in the conivaptan groups; the placebo group received a placebo loading dose. Measurements: Change in serum [Na+], measured by the baseline-adjusted area under the serum [Na+]-time curve (AUC), was the primary efficacy parameter. Secondary efficacy measures included the time from the first dose to a confirmed ≥ 4 mmol/l increase in serum [Na +], total time with serum [Na+] ≥ 4 mmol/l above baseline, change in serum [Na+] from baseline, and number of patients with a confirmed ≥ 6 mmol/l increase in serum [Na+] or normal [Na+]. Safety assessments included adverse events (AE), incidence of overly rapid correction of serum [Na+], and changes in vital signs and electrocardiographic and clinical laboratory parameters. Results: During the first 2 days of treatment, and over the entire 4-day treatment period, both conivaptan doses significantly increased the serum [Na+] AUC more than placebo (P < 0.01). Conivaptan 40 and 80 mg/day significantly improved all secondary efficacy measures. Conivaptan was generally well tolerated; infusion-site reaction was the most common AE. Conclusions: In hospitalized patients with euvolaemic hyponatraemia, i.v. conivaptan significantly increased serum [Na+] promptly and was well tolerated.