Residual ridge resorption begins following tooth extraction and continuously reduces alveolar bone volume, potentially creating a significant problem in dental implant treatment. In this study, the role of recombinant human bone morphogenetic protein-2 (rhBMP-2) in residual ridge resorption after tooth extraction was investigated. A polylactic acid/polyglycolic acid copolymer-coated gelatin sponge carrier was implanted with or without rhBMP-2 (1 microg) in the mesial root sockets after removal of maxillary first molars in male Wistar rats. Fine structural and histomorphologic analyses were conducted 3 to 84 days after implantation. Direct bone formation was first observed after 5 days on the rhBMP-2 side, which was transformed into cortical alveolar ridge with a smooth periosteal layer by 84 days, whereas the control side displayed slower healing. Bone histomorphometry revealed greater total bone area and increased bone height after 14, 28, 56, and 84 days on the rhBMP-2 side compared to the control side, and differences were significant after 14, 28, and 56 days. Larger numbers of proliferating cells and densely populated differentiating mesenchymal cells were observed on the rhBMP-2 side than on the control side in the early stage, and chondrogenesis was not observed. The findings indicate that rhBMP-2 may stimulate proliferation and differentiation of mesenchymal cells in the rat maxillary root socket to preserve cortical bone volume in the socket without any evidence of chondrogenesis.