Estriol has been showed to prevent bone loss in osteoporotic rats and postmenopausal women, but the mechanisms remain unclear. In the present study, we evaluated the effect of estriol on osteoblastic MG-63 cells in vitro, and compared its action with 17b-estradiol (E2). Cell proliferation was determined by measuring total cell numbers and [3H]thymidine incorporation. Cell function was studied by measuring alkaline phosphatase (ALP) activity and secreted osteocalcin. Our data showed that estriol stimulated MG-63 cells proliferation in a dose-dependent manner, but had no influence on ALP activity in MG-63 cells and osteocalcin production. Compared with estriol treatment, E2 showed a stronger proliferation. Estrogen receptor (ER) a and b expression in MG-63 cells can be detected by Western immunoblot analysis, and the proliferative response to E2 and estriol can be all abrogated by ER antagonist ICI 182,780. In conclusion, estriol stimulates osteoblastic MG-63 cells proliferation, but has no effects on differentiation. The proliferative response to estriol is mediated by the ER. These results suggest that estriol has an effect on osteoblastic proliferation, and this may contribute to its actions on prevention of bone loss.