The parabiotic experiments on a normal adult female rat connected with a castrated partner, described by Matsuyama and Yatsu, brought into evidence very important facts. The ovaries and uterus of the former presented a large hypertrophy, at times pyometra or hydrometra; enormous cystic ovarian follicles, or considerable luteinization were found. In cases of parabiosis of two normal females no noteworthy alteration was observed. Goto reproduced and confirmed the above experiments. Matsuyama did not draw any causal conclusion therefrom, and Goto admitted the existence of a “Kastrohormon, ” a substance which would be present, in a greater quantity, in castrated animals, without any further analysis as to its nature. Our present knowledge, mainly due to the works of Evans, Zondek and Aschheim, Smith and Engle, on the influence of the anterior hypophysis upon the gonads enables us to understand the phenomena above referred to: the prehypophyseal hormones which exist in greater quantity in the circulation of castrated animals are those which by their passage into a normal partner induce such effects to appear. In other words, the parabiosis of a normal animal with a castrated one (“C.M.” or “S.F.”) is* equivalent to a chronic treatment with hormones of the anterior lobe.