Electrolytic lesions destroying the basal tuberal region or the posterior median eminence of the hypothalamus affected the reproductive function in White Leghorn cocks. Degeneration of adenohypophysis, testicular regression and inhibition of comb growth were observed in these birds. It is supposed that these area have been the site of regulation center of reproductive function. Usually in the lesioned birds destroyed posterior median eminence testicular interstitial tissues strikingly developed containing large amount of cholesterol in the cytoplasm of the cell, whereas seminiferous tubules were regressed markedly. It appears that the nucleus of posterior median eminence has been related with the control of prolactin secretion in male chickens. Partial atrophy in the caudal lobe of adenohypophysis was observed in some lesioned birds destroyed the same area. The result indicates that the lesion of posterior median eminence caused the partial destruction of hypothalamic neurohumor tract to the caudal lobe of adenohypophysis. Destruction of the area ventrocaudal to the anterior commissure including the nucleus hypothalamicus posterior caused marked degranulation of some basophils which were supposed thyrotrophs in the cephalic lobe of adenohypophysis. This result indicates that ventrocaudal part to the anterior commissure is the site of regulation of thyroid stimulating hormone secretion. Destruction of ventromedial region including the nucleus hypothalamicus inferior caused marked increase of body weight, but there observed no histological changes in reproductive organs. Lesions in the preoptic area which have been supposed to regulate ovulation in hens were not correlated with testicular activity in cocks.