The possible functions of a light-induced electron transfer to oxygen in the photosynthetic electron transport chain of higher plant chloroplasts are considered. The thermodynamic preconditions, as well as the experimental data about the participations of ferredoxin, the components of photosystems I and II, and plastoquinone in oxygen reduction are examined. It is concluded that, even in the presence of ferredoxin and ferredoxin + NADP+, oxygen reduction is carried out mainly by the membrane-bound carriers of the photosynthetic electron transport chain. The hypothesis is put forward that most superoxides, which are produced by reduction of O2 molecules by the intramembrane components of the acceptor side of photosystem I, are reduced within the membrane by the plastohydroquinone molecules to the hydrogen peroxide. It is assumed that the H2O2 molecules that originate as the result of this process serve for signaling about the redox state of the plastoquinone pool.