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The purpose of this work was to examine the mammary glands of adult, ovariectomized female cynomolgus macaques (Macaca fascicularis) in a long-term study of the effects of hormone treatments on chronic disease. Treatments included conjugated equine estrogens (CEE), medroxyprogesterone acetate (MPA), CEE+MPA, and tamoxifen. Doses were scaled from those given women. Treatments were given in the diet for three years, followed by necropsy and tissue collection. Endpoints evaluated included glandular histology, histomorphometry, and immunohistochemical detection of the proliferation marker Ki-67, estrogen receptor (ER), and progesterone receptor (PR) in mammary epithelial cells. Major findings were as follows: CEE induced PR expression and focal to diffuse lobuloalveolar proliferation. Proliferation was increased by the addition of MPA, but was not induced by MPA alone. Tamoxifen induced ER and PR but not Ki-67 expression or glandular hyperplasia. Neoplasms were not seen. These findings indicate that progestogens may exacerbate, not antagonize mammary gland proliferation induced by estrogen replacement therapy, and that tamoxifen has both estrogen agonist and antagonist effects on sex steroid receptor expression in the normal primate breast.