Using the rabbit as an animal model, the changes of enzymatic activities of bone-specific alkaline phosphatase (ALP) in plasma and in the callus during callotasis were studied. Bone-specific ALP activities were measured by the wheatgerm agglutinin precipitation method. Distractions were performed on different groups of animals commencing on 3rd, 5th and 7th day after osteotomy. Bone-specific ALP activities in the plasma of all these groups dropped significantly on first few days compared with those values before osteotomy. However, when distraction was started, the plasma bone-specific ALP rose progressively until day 35 and resumed normal gradually until day 70. Immediate rises in enzymatic activities after distractions were observed in the 5th- and 7th-day treating groups but not in the 3rd-day treating group. The bone-specific ALP activities in the callus samples also increased until day 35 and gradually decreased. With the plasma bone-specific ALP as an indicator of the biological activities of the osteoblasts, the effect of external mechanical stimulation in a form of the tension stress on the biological tissue is well demonstrated in this study.