Arginine hydrochloride is used both diagnostically to test for growth hormone deficiency and therapeutically for treatment of metabolic alkalosis. We describe a 21-month-old girl who developed cardiopulmonary arrest following an accidental overdose of arginine hydrochloride. The patient developed acute metabolic acidosis and transient, but severe, hyponatremia. Thirty-six hours after successful resuscitation, she developed fatal central pontine and extrapontine myelinolysis. Unlike previous reports of arginine-toxicity, our patient showed no evidence of hyperkalemia. This case illustrates a previously unreported mechanism of arginine hydrochloride toxicity.