Background: During ventricular fibrillation (VF) only 39% of the variation in action potential duration (APD) is accounted for by the previous diastolic interval (DI_(n-1)) i.e., restitution, and the previous APD (APD_(n-1)), i.e., memory. We tested the hypothesis that a characteristic of the AP upstroke, the maximum rate of depolarization (Vdot_max ), also helps account for its APD. Methods: A floating microelectrode was used to make transmembrane recordings at 16,000 samples/s from the anterior left ventricular wall during four 20 s episodes of VF in each 6 pigs. Vdot_max, APD₆₀ and DI were calculated throughout all episodes. Stepwise linear regression was used to determine how well each APD₆₀(APD_60n) was predicted by Vdot_max of that AP, the four previous DIs _{(n-1, n-2, n-3, n-4)}, and the three previous APD₆₀s _{(n-1, n-2, n-3)}. Results: Vdot_max entered in the regression equation significantly more often (86% of VF episodes) than either APD_(n-1) (47% of episodes) or DI_(n-1) (58% of episodes). When these 3 variables entered 1st or 2nd, their coefficients were almost always positive, consistent with a longer APD associated with a (1) larger Vdot_max , (2) longer APD_(n-1) and (3) longer DI_(n-1). R² of the regression for all entered variables was 0.51±0.01 (mean±SD) . Conclusion: During the first 20 s of VF in swine, Vdot_max is a more important determinant of APD than is the previous DI (restitution) or the previous APD (memory). All variables together account for only one-half of APD variation during VF.