in the present study orthodontic force was applied to the molars of control group and experimental group treated with the lathyrogen beta-aminopropionitrile (BAPN). Observation resulted in the following conclusions ; 1) New alveolar bone formation in response to orthodontic force in BAPN-treated rats exceeded corresponding bone formation in control group when measured at two tension sites in the periodontal ligament. t 2) BAPN administration produced disorganization of the collagenous fibers of the periodontium of experimental group. . Multiple eosinophilic cell-free areas were found distributed throughout the radicular portions of affected periodontal ligaments. 3) The areas of periodontium surrounding orthodontically treated teeth exhibited relatively normal organization under these conditions, while the periodontium. of adjacent nonorthodontically treated teeth was disorganized. 4) The present results suggest that the typical histologic response to orthodontic force application can occur in the presence of a chemically and physically altered periodontium.