Increased release of pituitary prolactin as a result of placement of electrolytic lesions in the median eminence of the hypothalamus greatly accelerated the growth rate of carcinogen-induced mammary tumors in rats. Ovariectomy in these rats induced a rapid regression of tumors in spite of high plasma levels of prolactin. Grafting a pair of ovaries into ovariectomized rats with lesions of the median eminence did not alter the levels of plasma prolactin, but it did cause a rapid resumption of tumor growth. It is concluded that the prolactin stimulus to growth of mammary tumor is dependent on the presence of ovarian hormones.

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