In the hypophysectomized rat the intrasplenic ovarian graft (auto- and homoiograft) always becomes atrophic and never reaches any further development. Hypophysectomy has such an influence on the ovary, that this gland, after transplantation into the spleen of a castrated rat, takes only in 16% of the cases (the ovarian graft from and into non-hypophysectomized rats takes in 70 to 100 %). Hypophysectomy in rats with an already developed ovarian tumour, produces tumour regression. This facts prove that the hypophysis is the organ responsible for tumourigenesis in the intrasplenic ovarian graft. It is now necessary to prove that the hypophyseal gonadotrophins are the hormones which produce the proliferative and blastomatous changes of the intrasplenic ovary. In a future paper we hope to put forth more confirmative data regarding this hypothesis.