Pathogenesis of widened AP-SP complexes often noted in electrocochleo-grams (ECochGs) in cases of cerebeliopontine angle tumors remains unclear. As acoustic neuromas usually originate from the vestibular nerve, some changes in this nerve might be involved in producing the broadened wave-form. Effects of lidocaine on the AP-SP complex were investigated in guinea pigs by injecting this Compound into the root exit zone of the vestibular nerve, where the cochlear efferent fibers run together. The AP-SP complex in response to clicks and tone bursts was recorded at the bony wall of the scala vestibuli in the basal turn of the Cochlea. Inactivation of the efferent cochlear fibers resulted in a marked widening of the waveform, independent of the interstimulus intervals (ISIs). This broad wave was composed of a DC shift which was quite similar to the envelope curve of the acoustic Stimuli. Although the effect of activity of the efferent cochlear fibers on negative summating potential (SP) has not been reported in detail, the authors con-cluded that the broad waveform observed in ECochGs in cases with cerebeliopontine angle tumors was caused by enhanced negative SP due to inactivation of efferent cochlear fibers.