Transplantation of Brown-Pearce tumor of th e rabbit was attempted in the testicle and the influences of sexual hormones (Test. prop. and Hexestrol) to the metastases and the pathological changes of testicular tissue were studied. The animals divided into control group and gr o ups given sexual hormones. Metastases were rather severer in the groups given sexual h o rmones than in the control group. Histo l ogical examinations of the testicle revealed growth of large epithelial tumor cells and small round cells in stroma of the testicle in the control group. The growth of large epithelial tumor cells was observed to be marked in the group given Hexestrol, while the growth of small round cells was prominent in the group given Test. prop. Furthermore, necrosis and hemorrhage were observed everywhere in the in t rerstitial tissue of the testicle. From the above findings, it was emphasized that the tumor cells which composed of this tumor had two types (the large cell of the epithelial type and the small round cell) and by the administration of sexual hormones, especially Hexestrol, the growth of this tumor was hastened.