1. In dba mice ovariectomized at birth the vagina, uterus, and mammary glands showed a gradual recovery from the castrate state, and finally reached the stage which they presumably can attain only under the influence of estrogenic hormones. Tumors of the mammary glands developed in 37 animals, of 75 examined, between the ages of 14 and 28 months (3 adenomas and 34 carcinomas).
2. As ovarian regeneration had not taken place the probability that estrogen originated in some other organ in the absence of the ovaries is suggested.
3. The consistent nodular hyperplasia of the suprarenal cortex and close morphological similarity of cells of these nodules to lutein-like cells of the ovaries points to the abnormal suprarenals as possible sources of the estrogenic hormones.