It is a fact of great interest that when 131I-thyroxine and 131I-triiodothyronine are exogenously given, they are highly concentrated in the anterior hypothalamus, median eminence, and hypophysis. Schittenhelm and Eisler (1932) already found that the intravenous injection of 1.0mg/kg of thyroxine (T4) into dogs, cats and rabbits elevated iodine concentration in the tuber cinereum 10times. Since the introduction of 131I-T4, studies in this field have made a remarkable advance, and Harper and Mattis (1950, 1951), Jensen and Clark (1951), and Courrier et al.(1951), using this method, nearly simultaneously confirmed the results of Schittenhelm and Eisler (1932). Although this phenomenon was observed very distinctly in dogs and rabbits, it did not seem to occur so remarkably in all the animals. 131I-T4 concentration was found in the descending order, in the neurohypophysis, the stalk, and the median eminence. Ford et al.(1955) obtained a similar result with 131I-T3, and Courrier (1957) reported that 3,5',3'-L-T3 showed the highest affinity to these sites, followed by 3, 3'-L-diiodothyronine and T4. Taurog et al.(1956,1957) could not observe such accumulation of thyroid hormone in rats and guinea pigs. According to Ford et al.(1957), there is difference between 131I-T4 and 131I-T3 in the site of concentration, the latter being concentrated in the site where tissue oxygen consumption is higher. In experiments with rabbits, Ford and Gross (1958 a) confirmed that T3 was principally concentrated in Nucleus paraventricularis and the median eminence, and especially highly in the neurohypophysis. Ford and Gross (1958 b) further observed the concentration of T3 in the neurosecretory system in guinea pigs. These reports suggest that T3 and T4 are related, at a higher level than the hypophysis, to the hypothalamohypophysio-thyroidal system. Previously the authors extracted thyrotrophin releasing factor (TRF) from the anterior hypothalamus , including neurosecretory nuclei, which controled the hypophysio-thyroidal system, and was presumably related to exogenous T3 and T4. The relationship between the secretion of TSH and the concentration of T3 or T4 in the hypothalamus was therefore studied by the administration of non-labelled T3 and-T4.