We present a novel measurement for studying defibrillation mechanisms: the time course of changes in the size of the left ventricular (LV) cavity within 500 ms following defibrillation. Mechanical changes can be linked to electrical mechanisms via an understanding of excitation-contraction coupling. Eight mongrel dogs were internally defibrillated 5-50 seconds (including backup shocks) after the onset of 20 ventricular fibrillation (VF) episodes per animal. Two dimensional, short axis, LV cavity, ultrasound images were recorded at 30 frames per second just prior to inducing VF, during defibrillation and following the shock. Each frame was individually analysed to yield the LV cavity area as a function of time. Defibrillation shocks were followed by a highly reproducible phenomenon: (1) a dramatic and rapid increase in LV area, (2) a more or less prominent LV area plateau and (3) a decrease in the LV area. The peak fractional area increase ranged from 1.65 to 4.64 times larger than the baseline (LV area just prior to defibrillation), averaging 2.18±0.686. Successful shocks took significantly longer (p < 0.01) to return to 1.3 times the baseline (407±209 ms) than unsuccessful shocks (296±130 ms). Extrapolating to electrical mechanisms, our novel measurement demonstrates that defibrillation causes immediate relaxation and therefore suggests a significant role for de-excitation in defibrillation.