Tetrodotoxin or procaine was introduced into the scala tympani or scala media. The cochlear potentials—including the negative potential in the organ of Corti—were recorded before, during, and after the introduction. Tetrodotoxin at a concentration of 5 × 10−8and 1 × 10−7g/ml blocked the action potential but had little effect on the resting potential and cochlear microphonic, when it was introduced into the scala tympani. On the other hand, the cochlear microphonic and action potential were depressed by 0.05%–0.1% procaine introduced into the scala tympani, whereas the endocochlear potential showed a temporary increase. The mechanism underlying the generation of the receptor potential is discussed, and it is concluded that the production of the cochlear microphonic depends on the presence of potassium ions in the endolymph.