Hyperplastic and neoplastic lesions in the mammary glands, uterus, adrenals and pituitary glands of untreated aging female rats were evaluated. The most common lesions in the uterus, mammary glands and adrenal cortex were endometrial stromal polyps, fibroadenomas and nodular hyperplasias, respectively. A few malignant neoplasias were found in the uterus and mammary glands. The pituitary glands of the senile untreated rats had diffuse hyperplasia and/or hypertrophy of PRL, STH and ACTH cells. In 25 out of 52 rats (48%), pituitary adenomas were detected. Histological examination showed that the adenomas were made up of immature cells, a combination of ACTH and immature cells, prolactin cells, or intermediate zone cells. The cytological changes in the hypophyseal cell types in the senile rats were usually accompanied by spontaneous neoplasias of the mammary glands. The possible role of hypophyseal hormones in the neoplasias of the mammary glands and the adrenal cortex is discussed. The present study has also shown that the variation in the reproductive state of the aging female rats, as assessed from ovarian morphology, had no striking effect on the incidence or the severity of the hyperplastic/neoplastic lesions in the organs examined.