The origin of fibroblasts, their proliferative activity and roles in the early stages of periodontal regeneration were investigated in order to better understand the periodontal healing process in furcation defects of the beagle dog after guided tissue regeneration. Newly divided cells were identified and quantitated by immunolocalization of bromodeoxyuridine (BrdU) injected 1 hour prior to sacrificing the animals. The results were as follows : 1. During periodontal healing in horizontal furcation defect, three different stages, namely the granulation tissue, connective tissue, and bone formation stages, were identified on the basis of major types of cells and tissue. 2. In the early stages of periodontal regeneration, both the remaining periodontal ligament and alveolar bone compartment were the major sources. 3. The majority of BrdU-labeled fibroblasts were located at the following areas : 1) the coronal zone of the defect in case of the connective tissue formed on the root surface 2) the area within an 400 μm distance from the remaining bone level in case of the periodontal ligament 3) the area within an 100 μm distance from the bone surface in case of areas of active bone formation.. 4. The highly proliferative fibroblasts adjacent to bone surface played a or role in the formation of osteoblast precursor cells, whereas both paravascular and endosteal cells played a minor role in new bone formation. In conclusion, it was suggested that the fibroblasts in the remaining periodontal ligament and bone will play a major role in periodontal regeneration, whereas both paravascular and endosteal cells will play a minor role in new bone formation.