The pituitaries of aging male Lobund-Wistar rats were investigated by histology, immunocytochemistry and electron microscopy. In contrast to previous studies undertaken on aging rats of various strains, no lactotroph hyperplasia was found in the adenohypophysis of the present study. Lactotroph adenomas producing PRL were infrequent, whereas gonadotroph adenomas containing FSH and LH were common. The frequency of gonadotroph adenomas was decreased in germ free as well as food-restricted rats and in those in which the germ free state was associated with reduced food intake.