Abstract: Objective Our previous results showed that the pathological damage of the skin and tissue exposure to pressure could be caused by 3 h-compression in rats.Further observation was needed for the level of superoxide dismutase(SOD) and nitric oxide(NO) in serum or tissues in SD rats with pressure ulcer so as to screen the optimal frequency of repositioning and risk assessment markers for pressure ulcer. Methods Total 48 SD rats were divided into six groups(8 in each group):the control(untreated),and pressure for one hour(1 h),two hours(2 h),three hours(3 h),four hours(4 h) or six hours(6 h) followed by the same time interval.After 24h,blood and local tissues samples from compression area were prepared for SOD and NO determination by enzyme-linked immunosorbent assay(ELISA).And statistical significance analysis of variance for SOD and NO were tested by Student's t test. Results Compared with the control,lower activity of SOD and higher level of NO were found in 3 h,4 h and 6 h groups.And the differences were statistically significant(P<0.05) and time dependence.However,there were not statistical significances between the control and the 1 h or 2 h group(P>0.05). Conclusion A pressure duration for 3h is a threshold for the levels of SOD and NO above which pressure damage may occur.Therefore,SOD and NO may be regarded as the early-stage and noninvasive markers in risk assessment at pressure ulcer.