Summary Direct, alternating and radio-frequency currents of 40 microamperes or more were passed between silver wire electrodes introduced in various positions within the guinea pig's cochlea. Irreversible depressions of electrical responses, both microphonic and action potentials were produced. Microscopic examination showed that two important mechanisms were a) necrosis due to heating and b) formation of bubbles by electrolysis or by heating. Reversible physiological effects can also be produced by weaker DC polarization. Two details of the bony anatomy that may be of electrophysiological significance are described.