The main question for this investigation was whether the hypertrophic ovary which results from intrasplenic transplantation in castrated rats is really also stronglyfunctional. A secondary question was whether an endocrine organ can have an effect on a neighboring target organ without benefit of the systemic circulation as a connecting link.Specifically, if a piece of uterus be placed in the spleen of a castrated animal togetherwith functioning ovary, will the splenic uterus be stimulated? A total of 38 immature female rats and 7 adult females were employed. In 10 of the former at the time of castrationa picee of ovary and a segment of the uterus were implanted in the spleen. Thirty days later the splenic uteri were greatly enlarged, by contrast with the nearly atrophic uteri in situ. The difference in weight was highly significant (P<0.001). Histologic evidence of estrogenic stimulation was also apparent in the splenic uterus.