Eighty-two thymic tumors induced by N-nitroso-N-propylurea in inbred F344/DuCrj rats were examined by light and electron microscopy. These tumors were diagnosed as malignant lymphomas and classified according to light microscopic features into three types: 1) lymphoblastic (57%), 2) large cell (32%), and 3) pleomorphic (11%). Electron microscopy revealed no epithelial cells in all 82 malignant lymphomas, except for 2, in which one sheet of epithelial cells was found under the capsule. This finding confirmed that all of these thymic tumors were malignant lymphomas, not thymomas. The tumors of the lymphoblastic and large cell types consisted of lymphoid cells with a few macrophages. Lymphoid cells of the lymphoblastic type were medium sized and contained a moderate amount of polyribosomes and a few clustered dense bodies; cells of the large cell type were much larger than those of the lymphoblastic type and contained many more polyribosomes and larger nucleoli. The tumors of the pleomorphic type consisted of lymphoid cells with severely infolded nuclei and interdigitating reticulum cells that were thought to be nonneoplastic in nature. No viral particle was found in these cells among the three types of thymic lymphomas.