The purpose of the present study was to clarify wound healing process of periodontal tissue following experimental flap surgery, when applied the guided tissue regeneration (GTR) technique using a cross-linked atelocollagen membrane (AM). Mucoperiosteal flaps were made on the palatal gingiva of maxillary first molars of rats. The cementum was removed by curettage in order to expose the dentin surface. An AM was implanted into the site of dissection in the experimental group, while the control group received no implantation. The resorption processes of AM and wound healing processes of exposed root surface in the experimental group were examined by electron microscopic observation, 1, 3, 5, 7, 14 and 21 days, and 2, 3 months after the implantation. While the wound healing processes of periodontal tissue in the control group were examined at the fine structural level, 2 and 3 months after the flap surgery. The results were as follows. At the resorption processes of AM, the early invasion of a large number of neutrophils appeared in the site of of implantation. Neutrophils were attracted to and adhered to the AM fibers over the first few days after. AM was resolved to fine fibrous structure by the neutrophils between 1 and 3 days. A large number of macrophages appeared in the implanted site between 3 and 7 days, and neutrophils subsided after 5 days. The implanted material was rapidly resolved be macrophages with active phagocytosis, sometimes forming giant cells. Fibroblasts were invading to peripheral gingival connective tissue and were development of microfilament were observed. The implanted materials were completely resolved after 14 days. In light microscopical findings, of the experimental group, epithelial downgrowth was markedly inhibited, fibrous bundles of the gingival connective tissue were clearly arranged vertical to the root surface and new cementum tissue deposited to the root surface after 21 days. At electron microscopic observation in early stage of healing, dens granular layer (d. g. l.) was presented to the dentin surface. After 2 or 3 months of the control group, high density fibrous layer increased to the root surface. Connective tissue fiber bundles were paralleled to the root surface. The above results indicate that the GTR technique using an atelocollagen membrane may provide an effective method to promote periodontal tissue regeneration after periodontal surgery.