Objective To study the damage focused ultrasound inflicts on the membrane permeability of Ehrlich ascitic tumor (EAC) cells and the relationship between changes in membrane permeability and focused ultra-sound exposure time. Methods The relative survival rate of tumor cells was examined at various intensities and dif-ferent exposure times using focused 2.2 MHz ultrasound. The uhrastructure changes were evaluated with a scanning electron microscope after different exposures. Membrane permeability was investigated by incorporating fluorescein isothiocyanate dextran (FD5OO) , and membrane damage was evaluated by measuring lactate dehydrogenase (LDH) release. Results Morphological observation showed there were numerous microvilli on the surface of un-exposed cells. When the cells had been irradiated with focused ultrasound for 30 s there was only a slight effect on the shape of the cells and the number of microvilli was slightly reduced. When the cells were exposed to ultrasound for 60 s, the surface of many cells became relatively smooth with no obvious microvilli, and several small craters were seen on the surfaces of cells where the cytoplasm seemed to have extruded through the membrane. The cell membrane was seri-ously damaged by sonoporation. The loading of FD500 in the unexposed cells was only 0.21%. When the cells had been sonicated with focused ultrasound for 30 s or 60 s, the loading of FD500 increased to 11.46% and 18.50% re-spectively. The released LDH activities in the 30 s group and 60 s group were 2.94±0.02 and 3.28±0.04 U/L, respectively. The activities of LDH increaased as the focused ultrasound exposure time was prolonged. Conclusion Focused ultrasound may damage the cell membrane permeability of EAC cells, and the damage increases as the expo-sure time is prolonged from 30 s to 60 s.