Incidence of spontaneous neoplasms in F344 rats throughout the natural life-span.

The age-related incidence of spontaneously occurring neoplasms and degenerative diseases in the F344 inbred rat strain was established from the histologic examination of tissues from 160 male and 192 female rats kept throughout their natural life-span. The most common neoplasms were leukemias (25%), mammary tumors (females, 40.6%; males, 23.1%), pituitary adenomas (females, 35.9%; males, 23.8%), and testicular interstitial cell tumors (males, 85%). Various less common neoplasms were observed: thyroid interstitial cell tumors, adrenocortical adenomas, carcinomas of the genitourinary tract, representative central nervous system tumors, pheochromocytomas, and tumors of mesodermal origin including mesotheliomas, myoblastomas, fibromas, and fibrosarcomas. Multiple tumor types were found in 176 of the rats; metastatic tumors were uncommon. Degenerative diseases including myocardial degeneration and nephrosis were often observed. The incidence rate of these neoplasms and degenerative diseases generally increased with advancing age of the animals.