The ears of anesthetized guinea pigs were exposed to intense tones of 135, 545, 2000, or 8000 cps at sound pressures (at the eardrum) from 138 to 148 db re 0.0002 microbar. The electrical output was recorded by differential intracochlear electrodes before, during, and after the exposures.The injury caused by 8000 cps centers in the basal turn, by 2000 cps in the second turn, by 500 and 185 cps in the third and fourth turns of the cochlea. No single tone, at the intensities and durations employed, injured all of the cochlea.The “threshold” and also the maximum voltage of the cochlear microphonic are valid indices of the anatomical injury seen under the microscope if the proper test frequencies and positions of electrodes are used.The destructiveness of a tone does not depend on its frequency. Equally severe (probably permanent) injuries were produced by a one‐minute exposure at all four frequencies tested when the intensity level was about 144 db at the eardrum.