Safety Study of Vasopressin V2 Receptor Antagonist on Patients With Severe Chronic Heart Failure (AQUAVIT).

* Men or women with chronic heart failure (NYHA Class IIIB and IV). * Women must be post-menopausal or surgically sterilized; they cannot be pregnant or nursing. * Age 21 to 80 years. * Chronic heart failure of at least 3 months duration. For 2 months prior to screening, patients must have had symptoms at rest or on minimal exertion for more than 50% of the time. For 2 weeks prior to screening, patients must have had symptoms at rest or on minimal exertion for more than 90% of the time. * Patients must be receiving a diuretic and an ACE inhibitor (or an angiotensin II receptor antagonist) for the treatment of heart failure. * Patients may be receiving digoxin, a beta-blocker or spironolactone