The purpose of this study are to delineate the temporal and spatial changes of bone mineralization at the distraction gap and adjacent parent bone, and to investigate the effect of microaxial dynamization on regenerate bone healing and on development of regional osteopenia at the adjacent parent bone, during callotasis. Sixteen Korean adult mongrel dogs underwent bilateral tibial lengthening by callotasis. To the right hindlimbs, no dynamization was applied and served as control group (group I, and axial elastic dynamization was conffered to the left hindlimbs (group II). Bone mineral density(BMD) was measured by dual X-ray absorptionmetry(DXA) at immediate post-operative day, post-operative 12 day, 22 day, 36 day, 50 day, 65 day, and 85 day. Quantitative computed tomogram(Q-CT) was also taken after sacrifice to analyze temporal changes of mineralization pattern in the distraction gap. The following results were obtained; 1. In both group I and II, the BMD was lowest at the interzone in the distraction gap and increased linearly toward the corticotomy surface. This pattern did not change with time until the consolidation of the distraction gap, but the difference of BMD between the interzone and adjacent parent bone decreased with progress of consolidation. 2. During the distraction period, BMD increased progressively at the distraction gap and adjacent parent bone in both group I and II. During consolidation period, although BMD of the distraction gap still increased progressively, that of the adjacent parent bone decreased progressively in group I; the more distant from the corticotomy surface, the more decreased the relative BMD. in group II, the decrease in BMD of the adjacent parent bone was less than that in group I which was statistically significant in mid-consolidation period. In conclusion, the new bone in the distraction gap during callotasis appeared to form in bilateral direction with linear gradient from interzone toward corticotomy surface. Dynamization during callotasis not only stimulated regenerate bone healing in the distraction gap, but also prevented the occurrence of transient regional osteopenia at the distant part of the adjacent bone during midconsolidation period.