Abstract : PURPOSES: Analysing ventricular fibrillation (VF) rate and regularity at different sites and at different times may help understanding some of the mechanisms underlying VF in humans. METHODS: Twelve episodes of VF (19.4 ± 5.6 s) were induced during electrophysiological study in eight men (63 ± 14 years old). Calculation of dominant frequency (DF) by fast Fourier transform, short-time Fourier transform, and analysis of the pitch frequency [VF cycle length duration (CL)] were performed. For each episode, we analysed the 12 lead-surface ECG, three unipolar, 10 near-field, and three far-field bipolar recordings by means of three quadripolar catheters positioned at the right ventricular apex (RV apex), right ventricular outflow tract, and at the coronary sinus (exploring the lateral left ventricular epicardium) (LV). RESULTS: Fast and regular discrete activation covered the whole duration of every intracardiac recording, whereas surface ECG consistently displayed chaotic and fibrillatory pattern. DF (5.25 ± 0.64 Hz) was very similar on surface ECG recordings and in various intracardiac recordings. Intracardiac activation was rather regular during VF despite the fibrillatory process with very low SD of the CL. There were some significant inverse correlations between VF rate and VF regularity. Intracardiac sites displaying the fastest and most regular activations were those including the RV apex. VF rate and stability slightly increased over time. Finally, the parameters exploring the VF rate were found to be well correlated together, as well as parameters of VF regularity CONCLUSIONS: Human VF induced during electrophysiological study has a clear DF of activation and appears quite regular in intracardiac recordings. There is some spatial heterogeneity, which needs to be more detailed in order to localize possible driving sources. Fastest VF are the most regular. Rate and stability tend to increase during the initial phases of VF.