The electron fraction (upper plots), the electron-capture, and beta-decay rates (bottom plots) as a function of time till collapse are shown for two presupernova models: 15 M_⊙ (left) and 25 M_⊙ (right). In the electron fraction plots the dotted lines (blue) and dashed lines (red) were already shown by Heger et al. (2001a) and correspond to the old models of Woosley & Weaver (1995) and to the new ones of Heger et al. (2001b), respectively. Here we have added the solid lines, which results from assuming NSE and thermal weak equilibrium (solid black line) or dynamic beta-equilibrium (solid green line).