We have investigated the question: Do defibrillation thresholds (DFTs) vary during an episode of 'fibrillation,' and if so, what are the cellular mechanisms involved? Unstable microreentry ('fibrillation') and defibrillation (DF) shocks were simulated in a two-dimensional model of cardiac tissue on a massively parallel processor Connection Machine. The modelled tissue consisted of 16384 (128×128) 100 × 100 μm2 ventricular patches that obeyed modified Beeler-Reuter membrane kinetics. Reentry was initiated by stimulating the central region of the sheet during the repolarization phase of a passing planar wavefront. The DFT was then determined for several times following this premature stimulus. DFTs remained low (< 13 μA/cm2) until the initial figure-of-eight reentry degenerated into multiple unstable vortices (DFT ≈ 21 μA/cm2). Superimposed on this jump in DFTs was a small (±2.5 μA/cm2) quasi-periodic oscillation in DFTs whose average frequency correlated with the rotational frequency of the vortices. Therefore, we conclude that (1) there exists temporal variation in DFTs during fibrillation, (2) DFTs are lower during early fibrillation, and (3) DF shocks above ≈ 24 μA/cm2 are always successful.