The resting potential of the scala media (EP) in the first turn of the guinea pig's cochlea was altered by the application of currents. The EP as well as the cochlear microphonics (CM) and the summating potential (SP) were enhanced when the source electrode was in the scala media, whether the sink electrode was in the scala tympani or scala vestibuli. Using the scala media electrode as the sink for the current caused decreases in these potentials. When the EP change was sufficient to reverse the dc gradient across the reticular lamina, the CM reversed their polarity. A linear relationship exists between the changes in CM and EP. These results support the electromechanical theory of the production of microphonics.