Introduction: Studies evaluating changes in HRV preceding the onset of ventric ular arrhythmias using conventional techniques have shown inconsistent results. Time-frequ-ency analysis of HRV is traditionally performed using short-term F ourier transform(STFT). Wavelet transform(WT) may however be better suited for a nalyzing non-stationary signals such as heart rate recordings. Methods and resu lts: We studied patients with a history of myocardial infarction implanted with a defibrillator with an extended memory. The RR intervals during the 51 min prec eding ventricular events requiring electrical therapy were retrieved, and HRV st udied by WT and STFT. 111 episodes of ventricular arrhythmia were retrieved from 41 patients(38 males, age 64±8 years). Heart rate increased significantly befo re arrhythmia. There was no significant variation in low frequency/high frequenc y components(LF/HF) observed for the group as a whole, probably due to a great d egree of heterogeneity amongst individuals. A subset of 30 patients also had hea rt rate recordings performed during normal ICD follow-up. WT did not show any d ifference in HRV before arrhythmia onset and during control conditions. Conclusi on: Variations in HRV before onset of ventricular arrhythmias were not apparent in this large dataset, despite use of optimal tools for studying time-frequency analysis. Introduction: Studies evaluating changes in HRV preceding the onset of ventricular arrhythmias using conventional techniques have shown inconsistent results. Time-frequ-ency analysis of HRV is traditionally performed using short-term F ourier transform (STFT). Wavelet transform (WT) may but be be better suited for a nalyzing non-stationary signals such as heart rate recordings. Methods and resu lts: We studied patients with a history of myocardial infarction implanted with a defibrillator with an extended memory. The RR intervals during the 51 min prec eding ventricular There was no significant variation in significant befo re arrhythmia. There was no significant variation in significantly increased befo rerrhythmia. in low frequency / high frequenc y components (LF / HF) observed for the group as a whole, probably due to a great d egree of heterogeneity amongst indivi dual subset. A subset of 30 patients also had he rt rate recordings performed during normal ICD follow-up. WT did not show any d ifference in HRV before arrhythmia onset and during control conditions. Conclusi on: Variations in HRV before onset of ventricular arrhythmias were not apparent in this large dataset, despite use of optimal tools for studying time-frequency analysis.