The histologic responses of periapical tissues to root canal fillings with Grossman sealer were studied 10 dogs. Root canal fillings were performed on the 20 lower and upper teeth. The animals were sacrificed 1,2,3,4 and 5 weeks after the completion of operation. The following results were based on histopathologic studies; 1) After 1 week, the necrosis of dentin and cementum surrounding root apex was found in the root canal fillings. 2) After 2 weeks, the necrosis of cementum and surrounding alveolar bone were revealed. The resorption of dentin was appeared partially. 3) After 2 weeks, newly formed dentin was appeared surrounding necrotic dentin. 4) After 4-5 weeks, the osteoblastic activity was revealed abundantly surrounding the alveolar bone. 5) Fibrosis was prominantly appeared surrounding over-filled area, and fibrous encapsulation was performed.