Changes in the serum levels of anterior pituitary, thyroid, parathyroid, and adrenal hormones following subtotal thyroidectomy in 31 patients with Graves' disease were investigated. In 14 patients, rapid ACTH tests were performed on the preoperative and the first, third, and seventh post-operative days. Remarkable differences were not seen with regard to the changes in anterior pituitary hormones or cortisol, compared to those seen during general surgery. As to the thyroid hormones, the serum level of triiodothyronine (T3) decreased markedly after surgery and fell to half that of the preoperative value on the first postoperative day. Thereafter, a low value of T3 was maintained during the early postoperative period. Unlike T3, the serum level of thyroxine (T4) decreased gradually until the 7th post-operative day. The levels of both epinephrine and norepinephrine increased transiently during surgery, but the serum level of norepinephrine increased again on the third postoperative day. In the postoperative period, almost half the number of patients showed an inadequate cortisol response to rapid ACTH tests. It is suggested that the unique responses, such as the rise in serum norepinephrine or an inadequate response of cortisol to ACTH, or hypocalcemia, after subtotal thyroidectomy in patients with Graves' disease is largely due to the rapid decrease of T3 in the hypothyroid state, as was noted during the postoperative period.