The early tissue reactions in the periodontal tissues of the tension zones following the application of force (30gm) to the maxillary first molar teeth of the albino rats were studied by the light microscopy and electron microscopy. The increase of periodontal fibroblasts was evident, particularly in 1 day survival period. Osteoblast differentiation and new bone formation on the alveolar bone surface were occurred from 1 day survival period. Mononuclear phagocytes occurred consistently and in relatively high number adjacent to and at some distance from blood vessel Extensive breakdown of collagen fibers was observed. The increase of the phagocytosis of collagen by the active fibroblasts was evident. Also, collagen fibrils were sparse or lost near the macrophage.