Rates of electron-transfer (ET) reactions are dependent on driving force, reorganizational energy, distance, and the nature of the medium which the electron must traverse. In kinetically complex biological systems, non-ET reactions may be required to activate the system for ET and may also influence the observed rates. Studies of ET from tryptophan tryptophylquinone to copper to heme in the methylamine dehydrogenase-amicyanin-cytochrome c-551i ET complex, as well as studies of other physiologic redox protein complexes, are used to illustrate the combination of factors which control rates of interprotein ET reactions.