Cardioversion and defibrillation by a single high energy shock applied by myocardial or body surface electrodes is painful,  causes long term tissue damage, and is associated with  worsening long term outcomes, but is almost always required for treatment of ventricular fibrillation . As a initial step towards developing methods that can terminate ventricular arrhythmias painlessly, we aim to determine if  5/s pacing stimuli applied via  an implantable cardiac defibrillator (ICD) can modify human ventricular fibrillation. In 8 patients undergoing defibrillation testing of a new/exchanged intracardiac defibrillator,  five seconds of  pacing at five stimuli per second was applied during the 10-20 seconds of  induced ventricular fibrillation before the defibrillation shock was automatically applied, and the cardiac electrograms recorded and analyzed. The high frequency pacing did not entrain the ventricular fibrillation, but altered the dominant frequency in all 8 patients,  and modulated the phase computed via the Hilbert Transform, in four of the  patients. In this pilot study we  demonstrate that high frequency pacing applied via ICDelectrodes during VF can alter the dominant frequency and modulate the probability density of the phase  of the electrogram of the ventricular fibrillation.