Eight infants, 2 to 5 months of age, who were seen somnolence or irritability, seizures, and hypothermia are described. The symptoms developed following the ingestion of dilute formula. All infants were hyponatremic. Three patients were identified by the symptom complex and were evaluated prior to any therapeutic intervention. Plasma arginine vasopressin concentration and urinary osmolality were either normal or increased despite hyponatremia and decreased serum osmolality. These data, coupled with rapid biochemical and clinical improvement following fluid restriction and/or administration of 3% NaCl, strongly implicate the excessive release of arginine vasopressin in the pathogenesis of this syndrome of water intoxication.