AIM:Mesenchymal cells migrating towards fracture site and differentiating into chondrocytes and osteoblasts is the key process during fracture healing. This article is aimed to observe the changes of the expression of bone morphogenetic protein(BMP)-2 in the early stage of tibia fracture in osteoporotic rats so as to evaluate the reason of delayed fracture healing in osteoporotic rats. METHODS:The experiment was conducted at the Key Laboratory for Oral Biomedical Engineering of Ministry of Education from August 2005 to October 2006. ①Sixty-four 6-month-old female SD rats were randomly divided into ovariectomized(OVX) group and control group with 32 in each group. ②The rats in the OVX group underwent ovariectomy to establish type Ⅰ osteoporotic models. A small quantity of fat tissues was removed in the control group. Three months later,standardized tibia fracture was introduced. ③Eight rats were selected at days 7,14,21 and 28 after operation in the two groups. Some soft tissues around tibia and broken ends of fractured bone,bony callus,cortical bone and medullary canal were collected and treated with histological analysis,immunohistochemical staining,semi-quantitative reverse transcription-polymerase chain reaction(RT-PCR),respectively. RESULTS:①Histological analysis showed membrane bone formation and endochondral bone. A mass of chondrocytes appeared in bony callus of the OVX group at days 21 and 28 after operation. ②The results of immunohistochemical staining showed that mean absorbance(A) of BMP-2 was higher in the control group than the OVX group at days 7 and 14,but lower at day 21. ③The findings of RT-PCR revealed that BMP 2 level was higher in the control group than the OVX group at day 7,but lower at day 14. CONCLUSION:BMP-2 plays a key role in the early phase of bone callus formation during fracture healing. The declined and delayed expression of BMP-2 in osteoporotic rats may be an important reason for delayed fracture healing in osteoporotic rats.