For bridge-mediated electron transfer through time-dependent bridges it is shown that the superexchange mechanism may not be the only relevant tunneling mechanism. Electronic tunneling from donor to acceptor could also be mediated by a type of breakdown of the Born-Oppenheimer adiabatic approximation. A time-dependent effective Hamiltonian is derived for this process. (C) 2003 Elsevier Science B.V. All rights reserved.