The incidence and ultrastructure of resorption cavities were studied in albino rat molars. The transseptal fiber system between the 1st and 2nd maxillary molars in 30 albino rats aged 50 days was transected by buccopalatal incision. Fifty approximal surfaces represented the experimental material. Unaffected interproximal areas and approximal surfaces of adjacent teeth as well as of 10 teeth from five healthy, unoperated rats served as controls. The rats scheduled for operation were assigned to experimental groups each comprising three or four animals. Following injury, the incisional wounds were allowed to heal for 1, 2, 4, 5, 6, 8, 10, 14 and 21 days. The animals were sacrificed by an overdose of pentothal sodium and specimens prepared for light and electron microscopy. Normal, healthy teeth showed no resorption cavities on tooth surfaces facing the interproximal area, in contrast to the observation that minor resorption cavities with cemental repair were regularly present on root surfaces facing periodontal ligament. Following surgical injury, extensive resorption cavities in dentin and cementum developed on experimental surfaces adjacent to the area of wound healing. Arrest of active resorption in some of the experimentally induced cavities had occurred in 14-day specimens. Repair of resorption cavities appeared to start by attachment of connective tissue fibers to exposed dentinal and cemental collagen. The ultrastructural observations indicate that initial fibril attachment to exposed collagen is mediated by a granular, coating material resembling minute deposits of afibrillar cementum.