The cochlear microphonics and the auditory action potentials of guinea pigs, under anaesthesia, were recorded with differential electrodes. The ears were exposed to pure tones of frequencies 185, 545, 2000, or 8500 cps at intensities sufficient to cause severe acoustic trauma within one to six minutes. The cochleas were immediately fixed and subsequently studied microscopically. Good correlations were obtained between the immediate depression of electrical activity and the severity of anatomical injury. Electrical depression at any given position of electrodes in the cochlea was substantially constant regardless of the frequency of the test tone. Action potentials disappeared before the cochlear microphonic. Exposures above 145 db, measured at the animal's eardrum with a probe‐tube microphone, for more than half a minute regularly caused severe anatomical injury to the organ of Corti, often including gross rupture. Rupture begins at certain well‐defined points in the organ of Corti. The external hair cells are the most susceptible structures, but in the middle turns of the cochlea rupture may follow closely. The injury was greatest in the apical turns from 185 and 545 cps and in the basal turn for 8500 cps.