Background: Cycle length (CL) increases as ventricular fibrillation (VF) progresses. Objective: The purpose of this study was to test the hypotheses that increased CL is due to increased diastolic interval (DI), not increased action potential duration (APD), and that the DI increase is not solely due to increased postrepolarization refractoriness. Methods: In 10 swine, VF was recorded for 20 minutes using a floating microelectrode through a hole in a 504-electrode epicardial plaque. Mean APD, DI, action potential amplitude (APA), maximum change in voltage during the AP upstroke (V̇max), and CL were calculated from the floating microelectrode recordings each minute of VF. The refractory period was estimated from the minimum DI (DImin). In two animals, rapid pacing was performed to gauge refractoriness. Results: As VF progressed, CL, DI, and DImin increased (P <.05), whereas APD, V̇max, and APA decreased (P <.05). At 20 minutes, DImin was not different from mean DI at VF onset. Pacing captured, but 53% of paced wavefronts blocked within the plaque. Conclusion: Increasing CL in VF is due to increased DI and not APD, which shortens. The increase in DImin over time is much less than the increase in mean DI, indicating that the myocardium is excitable during much of the DI. This finding, along with the ability to pace at a CL shorter than the native VF CL and the poor paced wavefront propagation, suggests that the increase in DI is due not only to increased postrepolarization refractoriness but also to poor wavefront propagation because of decreased APA and V̇max secondary to global ischemia caused by VF. © 2009 Heart Rhythm Society.