With dark-adapted chloroplasts in which the plastoquinone was oxidized, a partial reduction of cytochrome b6 was obtained upon illumination with a pair of short saturating flashes. The second flash of the pair was much more effective than the first, and the reduction was inhibited by the system II inhibitor diuron. When the plastoquinone pool was reduced, both the reduction and the oxidation of cytochrome b6 were accelerated. The cytochrome b6 oxidation appeared to proceed in association with the reduction of cytochrome f, although these cytochromes are not simply connected in series. From these observations it is inferred that electron flow to the secondary donors of system I alternately caused the reduction and the oxidation of cytochrome b6. An interpretation is offered that also accounts for the transmembrane proton translocation that is associated with the oxidation of plastohydroquinone.