We report the case of a 4-year-old boy with a postoperative hypothalamic tumor, who exhibited unusual water and electrolyte disturbance. This developed as a late manifestation during the course of central diabetes insipidus (DI), which started when the patient was 2 years old. Clinically, hyponatremia and DI appeared alternately within 1 day. The hyponatremia (lowest value Na+ 115 mmol/I) was associated with afebrile convulsions. Assessment of fluid status revealed that the patient had a reduced capacity for arginine vasopressin(AVP) secretion (partial DI), which was not physiologically regulated and which was not concomitantly sufficient to produce maximally concentrated urine and allow the production of maximally diluted urine. This defective osmoreceptor function in association with the previously existing reduced capacity for AVP release seemed to be responsible for the fluid disturbance in the patient. The administration of nasal 1-desamino-8D-arginine vasopressin (DDAVP) only when urine output was increased, instead of regular administration at a fixed time, prevented both worsening of hyponatremia and development of DI.