Abstract 1. 1. A single microelectrode was used for simultaneously passing current pulses and recording bioelectric activity in single cells of the moth heart. 2. 2. The cell membrane responds to depolarizing and hyperpolarizing pulses as an ohmic resitor, showign no rectification with current flows up to 5 × 10 −9 A. 3. 3. A polarization resistance of 3.8±1 MΩ was measured and a specific membrane resistance of 1360Ω cm 2 was calculated. 4. 4. Action potential height was attenuated by depolarization and augmented by hyperpolarization. 5. 5. Although the slope of the prepotential declined in response to depolarizing pulses and increased in response to hyperpolarizing pulses, the rhythm of the spontaneous beat was not altered. 6. 6. The average rate of fall of the action potential was increased by hyper- polarization and diminished so markedly by depolarization that a plateau phase appeared where none had previously been detectable. The initial rising phase and the falling phase of the action potential could thus be altered by manipulating the membrane voltage.