Temozolomide, an alkylating imidazotetrazine derivative inhibits DNA replication and is used to treat aggressive brain tumors and other neoplasms including pituitary adenomas. The efficacy of temozolomide treatment is dependent on its ability to methylate the O6 position of guanine. The protein MGMT is able to repair this methylation, thus, temozolomide therapy is suggested in patients with little to no MGMT immunopositivity. Three surgically removed pituitary tumors were studied by electron microscopy before and after