The scala tympani in guinea pigs was prefused with mammalian Ringer‐Locke's solution, in which NaCl was replaced with either choline chloride or sucrose. Cochlear potentials were recorded from the basal turn before, during, and subsequent to the perfusion. Complete replacement of the perilymph withNa+‐free solution did not alter the endocochlear potential and the cochlear microphonic immediately, although the action potential was abolished rapidly. In most cases, the depression of the action potential was reversible. The mechanism underlying the generation of the receptor potential is discussed, and it is concluded that production of cochlear microphonic is not dependent on the concentration ofNa+in the perilymph but on the presence of highK+concentration in the endolymph.