An apparatus was developed that made it possible to push an electrode automatically with an absolutely even and smooth velocity through the cochlear partition of an anesthetized guinea pig. The ac and dc potentials of this perforating electrode were recorded during the perforation when sound was applied to the eardrum, or when an electrical generator was connected to the vestibular and tympanic channels of the cochlea. These measurements enable us to show that the endolymph is surrounded by an electrically insulating layer, and that the inner part of the organ of Corti is protected from external electrical potential fields. Further, it was shown that the microphonics produced by a vibration of the eardrum have larger values inside the endolymph than in the perilymph or near the round window. The microphonics seem to reach a maximum near the organ of Corti.The perforation measurements also made it possible to discriminate three types of tissues in the cochlea: (1) highly sensitive cells with large microphonics and dc potentials; (2) cells that do not produce large microphonics but still have in them large dc potentials; and (3) tissues that are electrically indifferent and can be represented by an electrical resistance. These distinctions and the measurement of the electrical conductivity of the endolymph permit a coarse description of the electro‐anatomy of the cochlear partition.