During the aging process modified functions of hypothalamic factors may cause sexually dimorphic changes in pituitary somatotrophs and lactotrophs. Pituitary tissue from young (4 weeks), adult (6 months), and old (20 month) male and female rates was labeled immunohistochemically for growth hormone and prolactin as well as ulrastructural study of the somatotrophic and lactotrophic cell populations were done. Quantitative immunohistochemical studies revealed that, the area % significantly affected with age in both sexes in somatotroph and lactotrophs. The unltrastructure of the somatrotrophs and lactrotrophs from old animals of both sexes evidenced changes suggestive of an immature state, with some cells having the appearance of cells undergoing an involutive process. We conclude that aging has a differential impact on the GH and PRL cells of male and female rates with respect to the immunohistochemical and ultrastructurl features of these cell populations.