The transplantability of granulosa cell tumors induced in mice by intrasplenic ovarian grafting was investigated. Female mice were ovariectomized and received autoplastic ovarian grafts into the spleens. The animals were killed at various periods from 2-12 mo. after grafting and the size of intrasplenic ovarian nodules was measured. There was an approximate linear increase in the size of intrasplenic ovarian grafts with passage of time. Intrasplenic ovarian tumors were transplanted s.c. into intact male mice. About 2/3 or more of intrasplenic ovarian tumors over 1.0 cm in diameter were transplantable. The larger the intrasplenic ovarian grafts, the greater was the estrous reaction in the vaginal smears. All animals bearing the grafts over 1.0 cm in diameter showed estrous reaction. Once intrasplenic ovarian grafts grow to the extent that estrogen is released with no complete inactivation by the liver, estrogens would escape into the general circulation, inhibiting excessive release of pituitary gonadotropins. Intrasplenic ovarian tumors probably become transplantable when they grow autonomously, even if the excessive release of pituitary gonadotropins is inhibited.