This study was to investigate the effects of mynol cement on the periapical tissues of 40 molars in ten dogs. Root canal fillings were placed with mynol cement and the animals were killed at different post operative periods ranging from 1 to 5 weeks. The teeth examined on the basis of microscipic findings. The following results may be drawn; 1. 1 week after root canal fillings, the necrosis of cementum and dentin including alveolar bone was to be seen. There were severe inflammatory changes in the periodontal ligament. 2. 2 weeks after root canal fillings, edematous changes and fibrosis in the periodontal ligament were revealed. 3. After 3-4 weeks, fibrosis was more prominent than 2 weeks. 4. 5 weeks after root canal fillings, the osteoblastic activity was found abundantly surrounding the alveolar bone.