A direct application of 7,12-dimethylbenz(a)anthracene (DMBA) to the ovary successfully produces an ovarian epithelial cancer in rat. To identify the histogenic process, the DMBA-treated rats were serially examined from the 15th to the 40th week following DMBA application. Furthermore, from the anticipated effect of estrogen on surface epithelial proliferation, the rats were put into an hyperestrogenic condition for immunohistochemical characterization of the tumor tissue. At the 20th week, the surface epithelial proliferations were seen to show remarkable multistratification with cellular pleomorphism. The proliferated surface cells began to infiltrate into the stroma to then form irregular gland structures. In immunoperoxidase stainings, both the adenocarcinoma cell and the surface cell has the same character in the immunohistochemical reaction for estradiol. On the base of these results, the induced cancer in rat was believed to have a similar histogenic process to the common epithelial tumors in human ovary.