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 (V̇max), also helps account for its APD. 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 of six pigs. V̇max, time from V̇max to 60% repolarization (APD60), and DI were calculated throughout all episodes. Stepwise linear regression was used to determine how well each APD60 (APD60n) was predicted by V̇max 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). V̇max 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 three variables entered first or second, their coefficients were almost always positive, consistent with a longer APD associated with 1) a larger V̇max, 2) a longer APD (n - 1), and 3) a longer DI(n - 1). R2 of the regression for all entered variables was 0.51 ± 0.01 (mean ± SD). During the first 20 s of VF in swine, V̇max is a more important determinant of APD than the previous DI (restitution) or the previous APD (memory). All variables together account for only one-half of APD variation during VF. Copyright © 2007 the American Physiological Society.