PURPOSE: The final goal of regenerative periodontal therapy is to restore the structure and function of the periodontium destroyed or lost due to periodontitis. However, the role of periosteum in periodontal regeneration was relatively neglected while bone repair in the skeleton occurs as a result of a significant contribution from the periosteum. The aim of this study is to understand the histological characteristics of periosteum and compare the native periosteum with the repaired periosteum after elevating flap or after surgical intervention with flap elevation. METHODS: Buccal and lingual mucoperiosteal flaps were reflected to surgically create critical-size, "box-type" (4 mm width, 5 mm depth), one-wall, intrabony defects at the distal aspect of the 2nd and the mesial aspect of the 4th mandibular premolars in the right and left jaw quadrants. Animals were sacrificed after 24 weeks. RESULTS: The results from this study are as follows: 1) thickness of periosteum showed difference as follows (P<0.05): control group (0.45+/-0.22 mm)>flap-elevation group (0.36+/-0.07 mm)>defect formation group (0.26+/-0.03 mm), 2) thickness of gingival tissue showed difference as follows (P<0.05): defect formation group (3.15+/-0.40 mm)>flap-elevation group (2.02+/-0.25 mm)>control group (1.88+/-0.27 mm), 3) higher cellular activity was observed in defect formation group and flap-elevation groups than control group, 4) the number of blood vessles was higher in defect formation group than control group. CONCLUSIONS: In conclusion, prolonged operation with increased surgical trauma seems to decrease the thickness of repaired periosteum and increase the thickness of gingiva. More blood vessles and high cellular activity were observed in defect formation group.