The upper limit of vulnerability hypothesis for defibrillation states that a successful defibrillation shock must both stop the fibrillation wave fronts on the heart at the time that the shock is delivered and not start new wave fronts that will lead to reentrant circuits being formed, causing the heart to refibrillate. Mapping studies have demonstrated that defibrillation shocks can halt all wave fronts on the heart but fibrillation will begin again with an initial activation pattern that is different from the activation pattern on the heart just before the shock is delivered. In a fashion similar to the reinitiation of fibrillation following a failed defibrillation shock, properly sized and timed shocks can be delivered to the heart during paced rhythm to induce funtional reentry that will initiate fibrillation. If the shocks are made incrementally larger, a shock level will be reached that is high enough not to start fibrillation in regular rhythm regardless of when it is delivered during the cardiac cycle. This shock level is called the upper limit of vulnerability. In this study, the formation of reentrant circuits with defibrillation-sized shocks and how this formation of reentrant circuits may be related to mechanisms of defibrillation, via the upper limit of vulnerability hypothesis, are discussed. © 1995, Churchill Livingstone Inc.. All rights reserved.