Chopping a defibrillation waveform into a series of high-frequency pulses may be a method to alter its effective shape. In seven pentobarbital anesthetized dogs, we measured defibrillation current (150) using a 10 msec long series of high-frequency (HF) pulses. HF waveforms consisted of monophasic rectangular current pulses at 1 KHz to 20 KHz with 50% “on-time” duty cycle. Control 150s were measured for 5 and 10 msec continuous pulses. Data were normalized by the current (2.61 ± 0.51 amps) required for the 10 msec controls. Resistance was the same for HF (71 Ω) and continuous (75Ω) pulses. 150 did not change with frequency (anova, p = 0.55) but all HF required twice (1.96 ± 0.17) the current of the 10 msec control. The 5 msec control required 1.27 ± 0.16 times the 10 msec control current. These data demonstrate that defibrillation was governed by the timeaveraged current since HF and 10 msec control waveforms required equal time-averaged currents but HF required twice the actual current and energy. Waveform duration was also important since HF waveforms were “on” for 5 msec but required 1.55 ± 0.16 times the 5 msec control current. These data also suggest that the heart defibrillates as though the current pulses were “filtered” with a time constant greater than 1 msec although no actual filtering was observed. Such filtering might help explain the role of waveshape on defibrillation efficacy and could be utilized in HF series to create arbitrary effective waveform shapes. We conclude that HF defibrillation is possible; that time-averaged current governs 150 for HF waveforms ≥ 1 KHz; and that HF chopping may permit waveform shapes to be effectively altered.