The effect of electrical stimulation of the crossed olivocochlear bundle (COCB) on cochlear potentials was studied in anesthetized and immobilized guinea pigs. Sound‐evoked responses and the endocochlear potential were measured simultaneously in the basal turn. The time course and the magnitude of the slow negative potential in the scala media produced by COCB stimulation can be modified by parameters of COCB stimuli. Augmentation of cochlear microphonic and inhibition of whole‐nerve action potential were found to be dependent on the slow negative potential in scala media and on parameters of the acoustic stimulus. The summating potential showed increase of the positivity with COCB stimulation. The role of the positive summating potential on the inhibition of action potential was discussed.