[en] The process of ovarian tumorigenesis in ddY/F and C3H/Tw mice was studied at various times until 18 months after whole body irradiation with a single dose of 130 R or 260 R of x-rays. The first ovarian tumors were observed at 9 months after irradiation in ddY/F mice. In a total of 28 of 71 ddY/F mice, 33 ovarian tumors were developed, which were subdivided to 25 luteomas, 4 granulosa-cell tumors, one mixed cell tumor and 3 tubular adenomas. On the other hand, 21 of 29 C3H/Tw mice had altogether 32 tubular adenomas and one mixed cell tumor. At older ages, small interstitial cells with eosinophilic cytoplasm were observed in irradiated ovaries. The cells resembled luteoma cells. Luteoma may be derived from the small interstitial cells. (auth.)