Though some biphasic waveforms significantly decrease the energy required for defibrillation, little is known about the effect of biphasic stimulation on the determination of other electrophysiological parameters in normal and infarcted hearts. To evaluate this, nine normal dogs and 12 dogs with myocardial infarction had activation threshold (AT), effective refractory period (ERP), strength-interval curves, and ventricular fibrillation threshold (VFT) determined with constant current stimulation to a pair of right ventricular plunge electrodes, and upper limit of vulnerability (ULV) and defibrillation threshold (DFT) determined with truncated exponential shocks delivered to a pair of wire electrodes coiled to contour the right and left ventricular epicardium. Each electrophysiological parameter was determined with a 5.5 msec monophasic and 5.5-msec biphasic (3.5 msec first phase) waveform. Though AT and VFT were not significantly different for the two waveforms, the ERP was significantly longer, the strength-interval curve shifted rightward, and the threshold for repetitive responses higher for biphasic stimuli. Compared to the monophasic waveform, the ULV and DFT were significantly decreased in a parallel fashion for the biphasic waveform. Neither the presence nor size of myocardial infarction significantly affected any of the measured electrophysiological parameters. In six additional dogs, sigmoid defibrillation probability curves were constructed from biphasic shocks of four energies including that of the DFT and ULV. The ULV energy predicted an effective dose that defibrillated 97% of the time (range 90%-100%). In conclusion, the increased defibrillation efficacy of the biphasic waveform is independent of its ability to activate fully repolarized myocardium and cannot be explained by a greater ability of biphasic waveforms to activate partially depolarized tissue. The parallel decrease in the ULV and DFT for biphasic stimulation and the finding that the ULV energy defibrillates with a high probability of success suggest similar underlying mechanisms for the ULV and defibrillation.