Two cases with pituitary tumour developed postoperative hyponatraemia which was not caused by inappropriate secretion of antidiuretic hormone. The one case with non-functioning macro-adenoma showed severe hyponatraemia (116 mEq/l) on day 11 after trans-sphenoidal surgery in association with diabetes insipidus (DI). The patients was treated by aqueous pitressin and saline administration to control urinary output and keep positive salt balance at the same time. The other case with GH-producing macro-adenoma showed progressive negative sodium balance with the total loss of 644 mEq resulting in hyponatraemia of 133 mEq/l. This was corrected by additional salt intake. The plasma atrial natriuretic polypeptide (ANP), antidiuretic hormone (ADH) as well as aldosterone levels were normal in the latter case. These patients were considered to manifest primary salt wasting disorder, which should be clearly differentiated from the syndrome of inappropriate secretion of antidiuretic hormone (SIADH).