The histogenetic process of experimental ovarian tumors induced by a chemically carcinogenic substance, 9, 10-dimethyl-1, 2-benzanthracene (DMBA) in rats was studied. Morphologically, the induced tumors can be classified into two groups : Adenocarcinoma and sarcoma. A particular pattern of histogenetic process in these tumors was evidenced by killing and following these animals at staged intervals from the initiation of the experiment. When treated with DMBA, an inflammatory change occurred around the region of a silk thread insertion; and subsequently, connective tissues proliferated. In the following stages, a layer of epithelial cells which constitutes glandular cysts of various sizes, on one hand and interstitial connective tissue on the other, proliferated around them, presenting the findings of fibroadenoma. This fibroadenoma provided a basic type from which two different histogenetic processes bifurcated. In one direction, cystic components dominated and proceeded toward adenocarcinoma, and in the other connective tissue proliferation prevailed to form a sarcoma type. Between the two of them, the process toward adenocarcinoma is of much interest because of its incidence clinically; and it was found that inclusion cysts appeared to play an important role in the developmental process, which should be of special interest when compared with various reports on the histogenesis of human adenocarcinoma.