ABSTRACT
Following results were obtained from the light microscopic and stereomicroscopic observations of the breasts of rats treated with 9, 10-Dimethyl-1,2-Benzanthracene(DMBA). 1) Adenocarcinomas developed in 17 rats (24%) among 70 DMBA-treated rats. 2) Terminal and buds (TEB) were observed longer in DMBA-treated rats than in control group, but they finally disppeared 4 monthes after treatment. 3) Many hyperplastic alveolar nodules (HAN) developed in DMBA-treated rats. 4) There were no transitional lesions between TEB and adenocarcinoma or HAN and adenocarcinoma. 5) The number of lobules was decreased in DMBA-treated rats. On the other hand, terminal ducts were increased in number. These findings suggest that DMBA stimulate the regression of lobules and induce to form terminal ducts from which adenocarcinomas and HAN develop independently.