In order to elucidate the mechanism of the Farquharson phenomenon, we studied the thyrotropin function in three groups of patients who had had high plasma levels of thyroid hormones for prolonged period of time : 3 patients (group A) had taken thyroid hormones for several years. One month after discontinuation of the treatment two of them were hypothyroid; in both of them plasma TSH did not increase after injection of TRH. Spontaneous recovery occured in those two patients respectively five and nineteen months after the end of the drug administration. One year after having stopped the treatment the third patient was apparently euthyroid; however, she was proved to have a complete TSH deficiency. Group B consisted of 27 patients who were studied after surgical ablation of toxic adenoma. Four of them were hypothyroid one to four months after surgery and had no response to TRH. Fifteen out of the 23 remaining patients were euthyroid but had such a lack of response to TRH from 5 days to one year after surgery. Group C consisted of forty three patients treated for Graves' disease and who had been euthyroid for at least one year following the treatment. Seven of them exhibited no or a subnormal response to TRH. Finally, thirteen patients (from groups A, B and C) who did not respond to TRH were given 10 mg TRH orally twice a day for two weeks. A normalization of the TRH test was then demonstrated in three patients and a partial recovery in three others. Thus, there is a frequent thyrotropin deficiency after prolonged high levels of plasma thyroid hormones which can disappear spontaneously or after chronic TRH administration.