Cases which present abnormality in water-electrolyte before and after operation of pituitary adenoma are occasionally reported. The authors have encountered a case in which neurological symptoms became aggravated abruptly with pituitary apoplexy after admission, hyponatremia was noted before operation and polyuria, not hypotonic urine was observed after operation. As a result of an endocrinological examination which may have an influence on water-electrolyte (ADH, aldosterone, ANP, etc.) the ADH level in hyponatremia before operation was high at 6.8 pg/ml; so, it was taken as SIADH. According to a study at the time of polyuria after operation, the ADH level was normal at 2.4 pg/ml, the ANP level was abnormally high at 140 pg/ml and the specific gravity of the urine was kept at 1.010 or more. So, polyuria was considered due to abnormally increased content of serum ANP. In polyuria due to abnormally increased content of serum ANP, the osmotic pressure of the urine is maintained relatively well, which is a clinical feature evidently different from diabetes insipidus. After operation for pituitary adenoma, water-electrolyte should be controlled with polyuria due to abnormally increased content of serum ANP in addition to diabetes insipidus taken into consideration.