A variety of neoplastic and degenerative lesions were observed in 216 aged male breeder (to 24 months of age) and virgin ACI/segHapBR rats sacrificed from 24 to 40 months of age. The most common neoplasms were pheochromocytomas, pituitary tumors, interstitial cell tumors of the testis, and tumors of the skin and subcutis. Many rats had multiple endocrine tumors. Age-related prostate hyperplasias and tumors were reported previously in these rats. Most of these tumors increased in incidence with advancing age, with few differences between virgins and ex-breeders. Testicular atrophy and tumors were more common in younger ex-breeder rats than in young virgin rats, but tumors reached a similar incidence in older virgin rats. Focal hyperplastic lesions appeared to represent the earliest stages of neoplasia in pituitary and adrenal glands, thyroid, prostate, testis, and liver. Common age-related nonneoplastic degenerative lesions were found in various tissues.