Estrogen/progestins replacement therapy prevents excess bone loss in post-menopausal women. The action of estrogen on bone is mainly through inhibiting bone resorption, but progestins exert its effect mainly through promoting bone formation. Recently osteoprotegerin (OPG) has been identified in osteoblast and displayed to inhibit bone resorption. The present study was performed to compare the action between E2 and P on OPG expression in cultured normal human osteoblast-like cells (hOB). OPG mRNA levels were determined by northern blot analysis, and OPG protein concentrations in conditioned media were determined by western immunoblot analysis and ELISA. After 24-48 h of treatment, E2 at 10(-8) M increased OPG mRNA levels and protein production in cultures of hOB, treatment with increasing dose of E2 caused a dose-dependent increase in the expression of OPG mRNA and protein by hOB. However, we also found out that after 12-48 h of treatment, progesterone at 10(-8) M had no effects on OPG mRNA levels and protein production in cultures of hOB, treatment with increasing dose of progesterone did not regulate the expression of OPG mRNA and protein by hOB. Thus, the data suggest that the different regulation of OPG production in osteoblasts by E2 and P may contribute to the mechanisms by which estrogen or progestins exerts its different action on bone resorption.