A 21-year-old woman underwent surgery for a pituitary tumor. On the 11th postoperative day, blood examination revealed severe hyponatremia, with a serum sodium level of 111 mEq/l, and two days later this increased rapidly to 137 mEq/l. On the 20th postoperative day, the patient developed dysarthria and gait disturbance. Head MRI on the 30th postoperative day demonstrated intense high-signal lesions in the pons and bilateral corpus striatum on FLAIR and DWI, and central pontine and extrapontine myelinolysis was diagnosed. The patient's symptoms improved gradually after rehabilitation and antispasticity treatment. It was suggested that the changes in serum sodium levels after pituitary surgery were due to impaired secretion of antidiuretic hormone due to degeneration of nerve terminals in the posterior pituitary. As pituitary surgery may trigger changes in serum sodium leading to myelinolysis, this possibility should always be borne in mind when treating such patients.