Morphological changes of DMBA-induced rat mammary tumors were observed following administration of prolactin and estradiol-17β or nafoxidine (anti-estrogen agent), and bilateral adreno-ovariectomy. 1) Following combined administration of prolactin 1 I.U./day and estradiol-17β2μg/day, all tumors demonstrated increase in volume. Their morphological changes were hypertrophy of tumor cells, increase of nucleo-cytoplasmic ratio, development of intracytoplasmic orgnelles, increase of free ribosomes and narrowing of the stroma. In addition, some tumors demonstrated marked secretion of fatty droplets and protein granules. 2) Following administration of nafoxidine 100μg/day, 90% of the tumors showed diminution in volume. Their morphological changes were smaller tumor cells in size, disappearance of mitotic figures, maldevelopment of intracytoplasmic organelles, decrease of free ribosomes, development of the basement membrane and marked proliferation of fibrosis in the stroma. On the other hand, myoepithelial cells demonstrated the least influences by anti-estrogen agent. 3) Following bilateral adreno-ovariectomy, 76.9 % of the tumors showed diminution in volume. Their morphological changes were necrotic changes of tumor cells represented by appearance of lysosomes and autophagic vacuoles. Myoepithelial cells remained relatively well and surrounded necrotic lesions. However, at last they disappeared and epithelial components of tumors were replaced by stromal components.