The influence of the ultrasonic surgical aspirator on the dura and underlying spinal cord was examined. Spinalcord-evoked potential was simultaneously recorded. In a test of identical time, the degree of influence on the dura, the pia mater, and the spinal cord increased according to the increase of energy of the ultrasonic surgical aspirator. With 60% energy at 20 seconds and 80% energy at both 10 and 20 seconds, spinal-cord-evoked potential showed wave changes. In light of these results, it is suggested that in using the ultrasonic surgical aspirator on the dura, the electrohistologic safety limit be set at 60% energy and the maximum time duration at one point less than 10 seconds.