Three hundred and seventy-three male mice were subjected to various oestrogenic compounds for periods of 50 days or more, in order to learn whether these compounds might affect the incidence of cancer. In each experiment, one drop of a solution (0·3 to 0·01%) of oestrogenic compound in benzene or alcohol was applied twice a week by means of a small paint brush to the skin of the interscapular region. This treatment causes pronounced changes in the coagulating glands which become converted into simple sacs lined by a squamous keratinizing epithelium, distended with keratinous debris, and frequently also with pus (Burrows and Kennaway, 1934; Burrows, 1935). At a later date the seminal vesicles undergo a similar transformation. These changes are shown in the appended illustrations. In twenty-two of the mice treated in this way, abnormal structures have been found either (1) just dorsal to the prostatic urethra, or (2) in the neighbourhood of the epididymis. These structures have consisted of single cysts in 20, of a collection of tubules in one, and of a single cyst and a tubular structure in another. In all instances the structures have been lined with squamous keratinizing epithelium.