In a national heart failure registry, hyponatremia (serum sodium < 130 mEq/L) was initially reported in 5% of patients and considered a risk factor for increased morbidity and mortality. In a chronic heart failure study, serum sodium level on admission predicted an increased length of stay for cardiovascular causes and increased mortality within 60 days of discharge. Hyponatremia in patients with congestive heart failure (CHF) is associated with a higher mortality rate. Also, by monitoring and increasing serum sodium levels during hospitalization for CHF, patient outcomes may improve. This review describes the pathophysiology of hyponatremia in relation to CHF, including the mechanism of action of vasopressin receptors in the kidney, and assesses the preclinical and clinical trials of vasopressin receptor antagonists--agents recently developed to treat hyponatremia. In hospitalized patients with CHF, hyponatremia plays a major role in poor outcomes. Vasopressin receptor antagonists have been shown to be safe and effective in clinical trials in patients with hyponatremia.