7,12-Dimethylbenz(a)anthracene (DMBA) 20mg was administered to 55-days-old Sprague-Dawley rats with gastric intubation. Two or three rats were sacrificed daily during the first week following administration of DMBA, then, weekly up to the 20th week. Mammary glands of each different stage were examined by the whole mount method, light microscopy and electron microscopy to observe tumorigenesis of rats mammary tumors introduced by DMBA. 1. On the 4th day following administration of DMBA, mammary epithelial cells at the peripheral portion of the duct demonstrated dilatation of endoplasmic reticulum and Golgi apparatus. On the 2nd week, an evidence of hypersecretion was well established. 2. However, on the 4th week, mammary epithelial cells showed atrophy and their intracellular organelles were regressed. On the other hand, myoepithelial cells showed cytoplasmic extensions to surround mammary epithelial cells. Stroma around the ductules also showed marked proliferation. 3. On the 7th week, mammary glands, examined by the whole mount method, demonstrated hyperplasty to form small nodules, namely, hyperplastic alveolar nodules (HAN). Among some rats, stroma around the ductules kept growing to be fibroadenoma. 4. On the 9th week, a tumor became palpable, and its histology was tubular adenocarcinoma. 5. Electron microscopic findings of carcinoma were increase of nucleocytoplsmic ratio, irregularity of nucleus, hypertrophy of nuclear membrane and enlargement of intracellular space etc..