A method for locating the facial nerve during removal of medium to large acoustic tumors is described. The method consists of stimulating the tumor mass with short rectangular pulses of monopolar, constant voltage and then presenting the electrical responses from the facial muscles through a loudspeaker. In this way, regions of the tumor where there is no nerve present can be identified and large parts of the tumor removed safely and quickly. Monitoring spontaneous electromyogram (EMG) activity was used to reduce the risk of damaging the facial nerve when parts of medium to large acoustic tumors adhering to the nerve were removed from 20 patients operated upon consecutively by the same surgeon. Only 1 patient lost facial function with the nerve intact anatomically. Nine patients had normal function immediately after the operation and had no detectable weakness upon discharge 6-8 days later, 4 patients had weakness which was resolved, and 6 patients had partial loss of facial function at the time of discharge. The same method may be used to identify the facial nerve during other operations in which this structure is at risk.