Estrogen, mainly estradiol (E2), and progesterone (P) are essential for the growth and differentiation of the breast, but their roles in breast cancer are highly debated. To understand how E2 and P influence cell proliferation and differentiation, it is essential to know how their receptors are regulated. Because of limited tissue availability, little is known about regulation of the two estrogen receptors (ERalpha and ERbeta) and the two progesterone receptor isoforms (PR-A and PR-B) in the normal human breast. What we know comes from rodent studies, which are not always pertinent for the human breast. We report now on regulation of gonadal hormone receptors during the menstrual cycle, pregnancy, and lactation in rhesus monkey mammary gland and on the relationship of these receptors to proliferation. We found that ERalpha but not ERbeta is down-regulated when E2 levels increase and when cells enter the cell cycle. PR-B but not PR-A is expressed in proliferating cells. Thus under normal conditions, the ratio of ERalpha to ERbeta in the breast depends on plasma concentrations of E2. Elevated expression of ERalpha (as occurs in postmenopausal women) is a normal response to loss of E2 and indicates nonproliferating cells. As selective receptor ligands become available, they will be helpful in delineation of the functions of these receptors.