Objective To investigate the expression and significance of nitric oxide synthase（NOS） during mandibular distraction osteogenesis and bone trauma healing, to futher study the mechanism of distraction osteogenesis. Methods Twenty-eight adult dogs were randomly divided into DO group（12 dogs）, acutely lengthening group（12 dogs） and control group（4 dogs）. Immunohistochemical examination were carried out to test the expression of NOS during the sixth day in distraction period, the second and eighth week of consolidation. Results In DO group infiltration of inflammatory cell was found in the distraction gap, more red blood cells（RBC） leak out around vascellum and matrix in the sixth day in distraction period. The expression of local iNOS（inducible NOS） and eNOS（endothelinal NOS） in DO gruoup and acutely lengthening group was higher than that in the control group（P0.05） in the eighth week of consolidation. Conclusion NOS as a sensitive index of tissue trauma are highly expressed, and RBC was found leaking out in the early of distraction, indicating that micro-trauma to some extent may occurr during DO procedure, the micro-trauma may be one of the significant factors which increasing regeneration of bone during DO．
