This is a randomized, double-blind, placebo-controlled, parallel-group forced up-titration study. Randomization will be stratified according to the patient's baseline serum sodium concentration (137-144 and <137 mmol/L). The dose of study drug will be increased to the next level on Day 15, and the total duration of the double-blind treatment period is 120 days.

• 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