An awareness of spontaneous lesions occurring in an experimental animal is essential in evaluating long-term experiments in that animal. This is especially crucial in gerontological studies. With this in mind rats from several strains at our institute have been set aside specifically to determine incidences of various lesions occurring during aging. The feamle WAG/Rij (Wistar derived) rat was the first animal to be investigated. The rats were allowed to complete their life-span, and 290 female WAG/Rij rats had an average life-span of 31 mo. (range 11-43). In these animals, a total of 519 tumors was seen histologically, almost 80% of which involved the endocrine glands. The most common tumor, adenoma of the pituitary, was found in 69% of all females. Two unusual tumors were a mesothelioma of the pleura occurring in a 39-mo.-old rat and an ameloblastic fibro-odontoma in the jaw of a 28-mo.-old rat. Tumors, and not degenerative or infectious lesions, were the main reason for the rat coming to necropsy.