The ultrastructure of the reversal line during alveolar bone remodelling was investigated in the rat. Surface bone remodelling along the periosteum of the mandible was induced by the extraction of opposing maxillary molars. With transmission EM the reversal line was seen to be composed of a superficial electron-dense amorphous layer and a deep filamentous layer at 7 d after extraction. The reversal line exhibited strong alkaline phosphatase activity and contained acid mucopolysaccharide. Scanning EM of the surface of the line, exposed by sonication in distilled water, showed papillary structures, the surface of which appeared granular and exhibited a crystalline appearance. The tips of collagen fibrils of new bone were attached to the top of the papillae in the front area of bone formation. It is suggested that the reversal line is involved in the coupling of bone resorption and formation.