This study will investigate the application of a vasopressin antagonist in the treatment of hyponatremia most likely caused by inappropriate AVP secretion. The population studied will include patients with euvolemic or hypervolemic hyponatremia.

Inclusion Criteria:

• Serum sodium levels less than or equal 130mEq/L
• Euvolemic or Hypervolemic hyponatremia

Exclusion Criteria:

• Clinical evidence of volume depletion or dehydration
• Untreated severe hypothyroidism, hyperthyroidism, or adrenal insufficiency
• Uncontrolled brady-or tachyarrhythmias requiring pacemaker placement or treatment