The purpose of the present study was to study histologically and histometrically the inhibitory effect of aspirin on root resorption induced by mechanical injury of the periodontal soft tissues in rats. Resorption lacuna in the root surface of the molar in animals given both mechanical injury and aspirin administration contained fewer odontoclasts and was smaller in length and area than that in animals given only mechanical injury. The result of the present investigation indicated that the administration of aspirin might suppress root resorption induced by mechanical injury of the periodontal soft tissues.