Hole-mediated electronic couplings, reorganization energies, and electron transfer (ET) rates are examined theoretically for hole-transfer reactions in DNA. Electron transfer rates are found to depend critically on the energy gap between the donor/acceptor states and the intervening bases-the tunneling energy gap. The calculated distance decay exponent for the square of the electronic coupling, beta, for hole transfer between GC base pairs (and pi-electron D/A pairs) ranges from 0.95 to 1.5 Angstrom(-1) in the model structures as the tunneling energy gap varies from 0.3 to 0.8 eV (which we argue is the range of energy gaps for GC oxidation probed in recent experiments). We show that the tunneling energy gap depends on the ET reorganization energy, which itself grows rapidly with distance for ET over 1-5 base pairs. Inclusion of the distance dependence of reorganization energies for these hole transfer reactions gives the tunneling rates an apparent decay exponent of similar to1.5-2.5 Angstrom(-1). We show that ET rates observed in DNA across one and two base pairs are reasonably well described with single-step ET theories, using our calculated couplings and reorganization energies. However, the computed single-step tunneling (superexchange) ET rates for donor and acceptor species separated by three or more base pairs are much smaller than observed. We conclude that longer-distance ET probably proceeds through thermal population of intermediate hole states of the bridging bases. Switching between mechanisms as distance grows beyond a few base pairs is likely to be a general characteristic of ET in small tunneling energy gap systems.