TRH test and T3 suppression test were performed on patients with Graves' disease who underwent subtotal thyroidectomy after treatment with antithyroid drugs for 2.5 approximately 5 months. On 43 or the patients, TRH test was performed before, 1 week after and 1 approximately 2 months after surgery and T3 suppression test was also performed in 1 approximately 2 months post-operative period. For other 3 groups of the patients TRH test was performed at 2 approximatley 6 months, 6 approximately 12 months, 12 approximately 24 months and 24 approximately 41 months after surgery, and T3 suppression test was also performed just after each TRH test. As to TRH test, the response was defined as positive when basal TSH value was less than 2.0 muU/ml and peak value was more than 6.2 muU/ml or the difference between basal TSH and peak TSH value was over 5 muU/ml following TRH (500 mug) injection. T3 suppression test was perfomed by measuring the 24-hr thyroidal uptake of radioiodine after daily administration of 75 mug of T3 for 8 days. The response was defined as positive when the value for 24-hr uptake after T3 administration was less than half of the control value. The results were as follows; (1) In 11 of 43 patients, response to TRH test already changed to positive 1 week after operation and in 21 of 43 patients TRH test changes to positive 1 approximately 2 months after operation. (2) In general, response to TRH test changed to positive earlier than response to T3 suppression test. (3) T3 suppression test in 1 approximately 2 months after operation was useful to evaluate prognosis. (4) A half of the positive respondents to TRH test showed exaggerated response. (5) Basal TSH value of positive respondents to TRH test was 9.27 +/- 1.81 muU/ml (mean +/- SE) which exceeded the normal range. (6) Some patients showed negative response to conventional T3 suppression test despite of their high basal TSH value. This might be due to the insufficient dose of T3 to suppress TSH. (7) Concerning patients whose serum T3-RU, T4, T3 and TSH were within normal limit after subtotal thyroidectomy, 80% of them showed correspondance in the results of TRH test and T3 suppression test.