Background: The influence of an increased left ventricular end-diastolic pressure (LVEDP) on the development of lethal arrhythmias in chronic heart failure is unclear. We investigated the effect of chronic and acute LVEDP increase on the epicardial activation time of sinus (SB) and paced (PB) beats. Methods: Six dogs underwent rapid ventricular pacing at 220-280 beats/min for 6-14 weeks for induction of heart failure. On the study day, baseline (ba) LVEDP was determined for the surviving heart failure animals (HF-ba), and for seven control animals (C-ba). The epicardial activation time (EAT, time between the earliest and latest epicardial activation) for five consecutive SB and five ventricular PB during the baseline hemodynamic state were recorded using a 504 electrode mapping-sock. In the control animals a 2-litre volume (vl) was infused over 10min to acutely increase the LVEDP (C-vl) to a level comparable to the chronic increased LVEDP of the HF-ba. The same volume challenge was performed in two HF animals (HF-vl) and the EAT for SB and PB was redetermined. Results: Three of six HF animals died during induction of heart failure. In the three remaining HF animals, chronic LVEDP increased from 6 ± 1 to 17 ± 10.8 mmHg (P = 0.07), EAT for SB increased by 68% compared to control animals (HF-ba vs. C-ba, P < 0.05). In contrast, in the control animals the acute rise in LVEDP from 6.8 ± 4.5 to 14.7 ± 6.2 mmHg P < 0.05), shortened the EAT for SB (C-ba vs. C-vl, P < 0.05). A similar decrease in EAT for SB caused by acute volume load was seen in the HF animals, but did not reach significance due to the small sample size (one of the three remaining HF animals died of spontaneous ventricular fibrillation before the volume load). Chronic LVEDP elevation significantly prolonged the EAT for PB from 72 ± 11 to 120 ± 31 ms (C-ba vs. HF-ba) while acute LVEDP increase had no significant effect on EAT for PB. Conclusion: Chronic HF increases LVEDP and prolongs EAT, while an acute increase in LVEDP shortens the EAT for sinus beats. A prolongation of EAT in heart failure may make the heart more susceptible to ventricular arrhythmias and electromechanical dissociation.