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type, including the morphologic appearance and biosynthetic activity, is diff erent. OB diff erentiation includes three distinct periods: 1) Growth (proliferation) and extracellular matrix (ECM) biosynthesis, 2) ECM development and maturation, and 3) ECM mineralization. During the period of active proliferation, many genes are expressed, such as cell cycle genes (c-fos, c-myc, histone) and those of extracellular matrix proteins (procollagen I, fi bronectin). This is followed by a stage of matrix maturation characterized by a high expression of bone alkaline phosphatase. When mineralization begins, genes for proteins such as osteocalcin, bone sialoprotein and osteopontin are expressed [8, 9]. The eff ects of 1,25dihydroxyvitamin D3 on OB have been well characterized in rats and human osteoblast cultures; in both cases, stimulation and inhibition have been described in genes related to the diff erentiation of osteoblastic phenotypes [10, 11]. It is well established that estrogens infl uence osteoblastic growth and diff erentiation [12, 13]. Nevertheless, the simultaneous action of both stimuli has not been described.