Introduction: This study investigated if action potentials can be prevented by electrical field stimuli of long duration. Methods and Results: The transmembrane potential was recorded by a double-barrel microelectrode during field stimulation given across a papillary muscle from 10 guinea pigs. After 10 stimuli (S) with a 200-msec S-S interval, a 400-msec square wave shock was given just before or after the end of the effective refractory period following the 10th stimulus through electrodes 1 cm on either side of the papillary muscles. Another two stimuli (S' and S') having the same 200- msec S-S interval were given during the shock pulse to test if the action potentials induced by these two stimuli could be prevented by the shock. The shock strength was increased until the shock field prevented the action potentials induced by the S' and S' stimuli. The resting membrane potential was -85.5 ± 2.9 mV. For shocks causing depolarization at the recording site, the field strength required to prevent S'- and S-induced action potentials was 1.5 ± 0.4 V/cm, which depolarized the transmembrane potential to -55.3 ± 8.9 mV and -58.1 ± 7.2 mV from the resting membrane potential at the time of the S' and S' stimuli, respectively. The strength of shocks causing hyperpolarization required to prevent S'- and S-induced action potentials was 5.0 ± 0.8 V/cm, which hyperpolarized the transmembrane potential to - 105 ± 6.5 mV and -115.6 ± 6.9 mV from the resting membrane potential at the time of the S' and S' stimuli, respectively. Conclusion: Both depolarization and hyperpolarization caused by an electrical field can prevent action potentials.