We discuss the transport of electrons through ferromagnetic tunnel jun
ctions. The spin-up and spin-down chemical potentials are different at
the insulator-ferromagnet interfaces by different amounts between the
parallel and the antiparallel configuration. As a result, the tunneli
ng probabilities fdr the spin-up and spin-down channels change differe
ntly as the external voltages are increased. There is a strong bias de
pendence of the magnetoresistance ratio, consistent with experimental
results.