Objectives To elucidate the molecular changes of bone collagen during the development of postmenopausal osteoporosis and to investigate the molecular effects of estrogen replacement.Methods An adult ovariotomy rat model was used. Type-Ⅰ collagen and matrix metalloproteinase-9 (MMP-9) expressions in bone tissues of rats treated by sham surgery (SH), bilateral ovariotomy (OVX) and OVX with estradiol (OVX-E2) were analysed at mRNA level by using dot blot technique. The distribution of mRNA of these two genes in bone tissues was studied by in situ hybridization.Results The expression levels of both type-Ⅰ collagen and MMP-9 in bone tissues of OVX rats were higher than those of SH group, while treated with estradiol, the expression of both genes declined to some degree. In situ hybridization showed that type-Ⅰ collagen mRNA located in osteoblasts, whereas MMP-9 was mainly expressed in osteoclasts, some lining cells on bone surface, and some mononuclear cells in bone marrow.Conclusions The reduction of high bone tover in osteoporotic bone tissues induced by estrogen replacement may result from alterations in gene expression related to bone formation and bone resorption. These alterations are consistent with the changes observed previously by histomorphometry and biochemical markers of bone metabolism on OVX animals and postmenopausal osteoporosis.