Transmembrane electron transfer in lecithin vesicles mediated by diheptylviologen was studied by stopped-flow and absorption spectroscopy. The reaction was initiated by addition of dithionite to the bulk phase which reduced viologen distributed over the outer vesicle interface and the bulk. The viologen reduced 2 equiv of ferricyanide in the vesicle interior. The observed reaction was of second order with respect to the viologen mole fraction, with a first half-life of approximate to 1 s for 1.5 mol % viologen in the outer monolayer of the vesicles. The reaction was found to proceed by transmembrane diffusion of the doubly reduced, uncharged viologen ((C-7)(2)V-0) formed by disproportionation of two viologen radicals ((C-7V+) This lends support to our suggestion that this mechanism is generally at work in microheterogeneous media. Observations on a longer time scale indicate that leakage of dithionite through the membrane may be induced by introduction of <10 mol % diheptylviologen in the membrane, Arguments against a high membrane permeability of the dicationic (C-7V2+ are presented.