A 12-month-old girl with end-stage renal disease secondary to primary oxalosis was erroneously given an overdose of sodium chloride (400 mEq NaCl over 12 h) to treat hyponatremia. She became lethargic and hypotonic with signs of intracellular dehydration, and laboratory values revealed severe hypernatremia and hyperchloremia. Since hypernatremia was acute and development of intracellular idiogenic osmoles was presumably minimal, serum sodium was lowered rapidly over 14 h by hourly peritoneal dialysis using a commercial dialysate. This method of treatment proved to be safe and the patient survived without any short or long-term neurological sequelae.