The effect of orthodontic force on the collagenase and phosphatase activities of the adjacent alveolar bone was evaluated. Maxillary canines of male cats were treated orthodontically with closed coil spring so as to exert about 80g force. Sixteen cats were equally divided into one control group and seven experimental groups (12 hrs, 24 hrs, 36 hrs, 2 days, 3 days, 5 days and 7 days after orthodontic treatment). After sacrificing all animals on experimental intervals, collagenase, acid phosphatase (ACP) and alkaline phosphatase (ALP) activities were determined in the pressure and tension sides of alveolar bones. ACP activities increased in both the pressure and tension sides, but significantly increased in the pressure side continuously. ALP activities increased in the tension side at early stage (1-2 days after treatment), but changed small amount in the pressure side. Collagenase activities increased in the pressure side, especially at late stage (5-7 days after treatment). These results suggest that (1) orthodontic force increases the ACP, ALP and collagenase activities generally and (2) activities of ACP and collagenase increase in the pressure side, but that of ALP in the tension side and (3) activities of ACP and ALP increase at early stage, but that of collagenase at late stage after orthodontic treatment. Therefore it is shown that there are time differences in the formation and destruction of organic and inorganic components in the bone metabolism of alveolus with application of the orthodontic forces.