The effect of DDAVP (1-deamino-8-D-arginine vasopressin), a synthetic analogue of vasopressin, was studied in twelve patients with acute postoperative cranial diabetes insipidus (D.I.). The most severe D.I. occurred in four cases following total removal of tumor (3 pituitary microadenoma, 1 dermoid cyst). The urinary volume over 1000 ml per hour in these four cases could not be controlled by DDAVP but could be controlled by drip infusion of aqueous pitressin (AP) and pitressin tannate in oil (PTO). DDAVP was effective when the urinary volume was decreased in under 500 ml per hour. The mild D.I. occurred in four cases after partial removal of tumor (3 craniopharyngioma, 1 pituitary microadenoma). These four cases could be controlled by drinking water only during one or two postoperative weeks. DDAVP was administered in doses of 10 to 30 microgram two times daily after 2 or 3 postoperative weeks and the urinary production was normalized. The four patients developed D.I. after removal of functioning pituitary microadenoma operated by transsphenoidal route. These four cases were treated with drip infusion of AP and PTO during one or two weeks after the operation and were effectively treated with 5 to 15 microgram of DDAVP intranasally every 8 to 12 hours one or two weeks after the operation. Nine cases in 12 cases with postoperative D.I. became chronic D.I. The maintenance dose of DDAVP gradually lessened in accordance with decreasing urinary volume except the two cases of craniopharyngioma. No side effect was experienced for 19 months of treatment.