To test the “variable‐resistance” theory of production of cochlear microphonics, the impedance variation between scala media and the neck of guinea pigs was measured during low‐frequency sound stimulation. Results show an impedance variation at the sound frequency that can be as large as 100 Ω (1% of the dc impedance) occurring in the walls of the endolymphatic space. This supports the concept that the cochlear microphonic potentials are produced because of resistance change at the reticular lamina.