When the ear is stimulated by a steady tone scala media and scala vestibuli become less positive electrically relative to scala tympani. This ‘summating potential’ (SP) is a d.c. change related to the root-mean-square of the acoustic pressure, integrated over one or two waves. It increases up to injurious sound pressure levels. It is increased by additional positive polarization of scala vestibuli or media. It is modified and may even be reversed in sign by hydrostatic displacement of the cochlear partition. It broadly resembles the cochlear microphonic (CM) but is more resistant to most drugs and anoxia. The negative SP depends on the integrity of the internal hair cells. The external hair cells produce CM and may also generate small SPs, usually positive in sign. The SP generated by 7000 cps tone bursts is strong in the basal turn while those by 2000 and 500 cps are very small here but are strong in the second and third turns, respectively. The theory is proposed that the negative SP is the response of the internal hair cells, an amplifier action intermediate between a mechanical detector action of the cochlear partition and the excitation of nerve impulses. Both CM and SP depend on bending of the ‘hairs’ of the sensory cells in the proper direction.