The clinical and pathological course of 25 mammary tumors in rabbits has been described. The antecedent breast history and morphology of the growths allowed a natural classification into two distinct types, one of which was distinguished by a preexisting cystic mastitis and a papillary structure, while the other originated in clinically normal mammary tissue and was characterized by an adenomatous structure. The two types of neoplasia occurred almost exclusively in two family groups and heredity played a fundamental rôle both in the occurrence of the tumors and in the determination of tumor type. Endocrine changes, comparable with those found in animals after long continued administration of estrogenic substances, occurred in tumor-bearing rabbits and it was inferred that the spontaneous growths represented a natural counterpart of the experimental induction of neoplasia with estrone.