In comparison to the incidence in the mouse, mammary cancer in the rat is uncommon. Curtis, Bullock and Dunning analyzed the data on the occurrence of spontaneous tumors in 392 of 31,868 autopsied rats of a pedigreed colony maintained in the Department of Cancer Research of Columbia University. In addition to 65 benign mammary tumors and 2 adenofibromata with sarcoma, keratinizing cystadenocarcinoma of the breast was observed in 2 (0.02 per cent) of 8,956 animals in the general population over the minimum tumor age. The tumor bearers were females of the Fischer and Marshall strains. Bryan, Klinck and Wolfe described an Albany strain of rats with a high incidence of breast tumors. Of 51 females, 25 had fibroadenoma or fibroma, and 1, approximately seventeen months of age, an adenocarcinoma with foci of squamous metaplasia and sarcoma.