The present study was carried out to investigate histologically and histometrically the effect of fluoride on root resorption induced by mechanical injuries of the periodontal soft tissues in rats. Resorption lacuna in the root surface of the molar in animals given both mechanical injuries and fluoride administration was significantly smaller in length and area than that in animals given only mechanical injuries. Moreover, resorption lacuna in the former animals contained fewer odontoclasts than that in the latter. The results of the present study might suggest that the administration of fluoride suppressed root resorption induced by mechanical injuries of the periodontal soft tissues.