Osteoblast-derived matrix metalloproteinases (MMPs) are considered to play a crucial role in bone formation and initiation of bone resorption by degrading the bone matrix. MMP-2 is con-stitutively secreted in a latent zymogen by osteoblasts, and re-quires the process of activation mediated by membrane-type matrix metalloproteinase-1 (MT1-MMP) ftissue inhibitor of me-talloproteinase (TIMP-2) complex in the cell surface. Bone is one target tissue for progestins. In the present study, we ob- served the effects of progesterone on proMMP-2 activation and MT1-MMP expression, and alsoTIMP-2 levels in osteoblastic MG-63 cells. Gelatin zymograms and ELISA showed that progester-one have no effects on proMMP-2 activation. Using Western im-munoblot analysis, we unexpectedly found that treatment with increasing doses of progesterone in MG-63 cells caused a dose-dependent increase in expression of MT1-MMP protein, and after 48 h treatment, progesterone at 10-8 M increased MT1-MMP protein level. Confocal immunohistochemistry analysis also confirmed that progesterone induced MT1-MMP expression in MG-63 cells. The results of Northern blot analysis showed that progesterone at 10-8 M increased MT1-MMP protein levels after 48 h treatment. We also found that TIMP-2 levels were unde- tectable in MG-63 cells. In conclusion, progesterone increases MT1-MMP protein and mRNA levels in MG-63 cells, but has no effects on proMMP-2 activation, which is partly attributable to the undetectable levels of tissue inhibitor of metalloproteinase-2 (TIMP-2). Our studies suggest that TIMP-2 is involved in proMMP-2 activation, and regulation of MT1-MMP by progester-one may contribute to its actions on bone formation.