Conivaptan is a nonspecific arginine vasopressin receptor antagonist that has been used as therapy in adults who have hypervolemic hyponatremia due to congestive heart failure. Its use in children with congestive heart failure has not been reported. We describe the use of conivaptan in a 4-month-old infant girl with severe hypervolemic hyponatremia and heart failure. A therapeutic weight-based dose was extrapolated from the adult dose. Conivaptan therapy was administered for 48 hours, after which the patient recovered from her hyponatremia without untoward effects. Arginine vasopressin receptor antagonists such as conivaptan may be useful as therapy for hyponatremia associated with heart failure. Further studies are required before conivaptan can be recommended for routine use in children.