Aim [WT5”BZ]To study the changes in activity and mRNA expression of nitric oxide synthase (NOS) in different phases of atherosclerotic lesion.[WT5”HZ] Methods [WT5”BZ]24 male New Zeland white rabbits were divided into normal control group(6 rabbits) and hypercholesterol group(18 rabbits). At the time of the 8th, 12th and 16th week, 6 rabbits of hypercholesterol group were sacrificed. Using NADPH-diaphorase histochemistry, we studied the activity of NOS. The NOS mRNA expression in aortas in the 16th week was also studied with in situ hybridization.[WT5”HZ] Results [WT5”BZ]In the 8th week, in cholesterol group, there were three different aspects of NADPH-d stains in plaques, which were doubtful positive, positive and strong positive. In the 12th week, there were extensive NADPH-d moderate positive stains in plaques, particularly at the border of intima and media. In the 16th week, the activity and mRNA expression of NOS in both intima and media decreased accompanying with the forming of necrosis core.[WT5”HZ] Conclusion [WT5”BZ]Changes in activity and expression of NOS in atherosclerotic lesion are dynamic with different locations of cells.