Thyrotrophin releasing hormone (TRH) tests were performed at 4 or 8 weeks intervals, after the initiation of anti-thyroid treatment in 15 patients with Graves' disease. All TRH test were negative as long as the serum levels of thyroxine (T4) and triiodothyronine (T3) were elevated, and normalization of the serum levels of these hormones always occurred before the response to iv TRH was restored. In 13 patients the time from the patients for the first time were registered as biochemically euthyroid varied from 0-9 months (mean 3.1 months), before TRH response was restored. Two patients were still TRH non-responsive at the end of the study, even though they had been biochemically euthyroid for as long as 17 and 18.5 months. The TRH test, therefore, is not helpful in the evaluation of the effect of anti-thyroid treatment in patients with Graves' disease. There was an increase in the serum level of (TSH) from 3.4 +/- 0.3 (SEM) to 4.3 +/- 0.5 (SEM) ng/ml (P less than 0.05), and a decrease in the serum level of total T4 from 19.4 +/- 1.1 (SEM) to 5.8 +/- 0.8 (SEM) microng/100 ml in 13 patients from the first examination until the last time they were examined before restored TRH response. This finding shows that the pituitary gland has retained its ability to synthesize and secrete TSH even though no TSH could be released by iv TRH. In 6 TRH non-responsive patients with Graves' disease, serum TSH levels were suppressed from 2.5 +/- 1.2 (SEM) ng/ml before the administration of a single dose of 3 mg T4 orally, to 0.9 +/- 0.2 (SEM) ng/ml, 7 days after the T4 administration. Thus, the negative feed-back effect on the pituitary gland of the thyroid hormones is operating in these patients. This finding indicates that the TRH non-responsiveness in euthyroid patients with Graves' disease is not due to pituitary depletion of TSH, since the negative feed-back effect of the thyroid hormones is operating normally.