An apical periodontitis experimental model was produced by means of opening the pulp chamber of the mandibular first molar in Wistar strain rats. In particular, the behavior of bone tissue in the vicinity of the root apex was investigated histochemically, ultrastructurally, and quantitatively. In addition, in order to demonstrate the effects of prostaglandin on the formation process of apical periodontitis, we examined the effects of indomethacin on bone remodeling during experimental apical periodontitis. These experiments suggested that prostaglandin may stimulate osteoclastic bone resorption and that the relationship between bone resorption and formation in apical periodontitis is a coupling phenomenon.