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A great deal of works have been made in an attempt to establish the relation-ship between thyroid function and the hypothalamo-hypophysial pathway (for reviews, see D'Angelo, 1963; Reichlin, 1966). One experimental approach to this problem is an alteration of thyroxin level which, in turn, has been known to alter the amount of TSH output. If the discharge of TSH is under the control of the hypothalamus, any changes in the hypothalamic region should be recognizable. Along this direction, a series of experiments has been carried out in the ra