This histopathological study shows that Mastomys develops a wide variety of neoplastic and nonneoplastic lesions with age. In comparing neoplastic lesions of Mastomys with those generally found in mice and rats, Mastomys is more or less unique with respect to the development of lymphoepithelial thymomas (40%), parathyroid adenomas (11%), prostatic adenocarcinomas (5%), and gastric carcinoids (4%) and the absence of brain, lung, and mammary tumors. Of the nonneoplastic lesions, prostatic (38%), thymic (12%) and parathyroid (11%) hyperplasia, and moderate to severe generalized degenerative joint disease (96%) occur rarely in mice and rats. Within the limits of this study, in which the age of the animals ranged from 18 to 39 months, a clear-cut age-related pattern was seldom found for most of the lesions occurring in Mastomys.