：Objective To assess theinfluence of pre-exposure to non-ablating ultrasound on the effect of high intensityfocused ultrasound(HIFU)ablation.Methods Forty rabbits with transplanted VX2 tumors ontheir livers were divided into three pre-exposure groups(60 W,80 W and 100 W)and a controlgroup(O W),with 10 rabbits in each group.Each group was pre-exposed to lower intensityfocused ultrasound at the corresponding power.From each group,5 rabbits were randomlyselected to be exposed to HIFU on the next day.Each group received 14scans.The rabbitswere sacrificed 24 h later to take tissue samples for tfipheny tetrazoliumchloride(TTC)staining to measure the amount of coagulative necrosis.Hematoxylin andeosin(HE)staining and succinate dehydrogenase (SDH)were also measured to observe thestructure and activation of pre-exposed tumors.Results After HIFU exposure,the ablatedvolumes increased with pre-exposure acoustic intensity,and all volumes were larger thanthose in the control group.The ablating efficiency in the 100 W group was the highest.Thepre-exposure did not itself ablate tumor tissue,but in the 80 W and 100 W groups thestructure and activation of the tissues changed.Those in the 60 W group were not obviouslyaltered.Conclusion Prior exposure to non-ablating ultrasound can highly enhance theefficacy of HIFU ablation.