High incidence pulmonary tumors induced in the mice by diethylnitrosamine were studied as to their morphology and pathogenesis. Under the light microscope, the induced pulmonary adenomas may be divided into three types: solid, papillary and mixed adenomas. Ultrastructural study showed that some adenomas were derived from the Type II pneumonocytes, some from Clara cells and more than a half of the adenomas possessed the common features of both. The author terms the latter bi-phase differentiated adenoma, which is characterised by the presence of a lot of the lamellar bodies and electron dense granules in the cytoplasm as well as transitional from between the two. The bi-phase differentiated adenoma is firstly considered as originating form the Clara cells, secondly from the anaplastic cells in the bronchiolar epithelium. Ultrastructural study indicates that the lamellar bodies are probably derived from the multivesicular bodies and/or electron dense granules. The malignant change of the adenomas goes into malignancy resembling the bi-phase differentiated adenomas in the ultrastructure. The lung carcinomas of the mice are similar to those which occur in the human being.