The photosynthetic electron transport chain (PETC) is the principal place of appearance of reactive oxygen species (ROS) in plants under illumination. The peculiarities of this process in different segments of the PETC are discussed. Oxygen uptake observed under impaired electron donation to photosystem II is attributed mainly to hydroperoxide formation by reaction of oxygen with organic radicals generated after detachment of electrons by P680(+). Oxygen reduction in the plastoquinone pool is suggested to start with the reaction of O(2) with plastosemiquinone, and to be followed by reduction of superoxide to hydrogen peroxide by plastohydroquinone. The distribution of plastoquinone throughout the thylakoid membrane interior provides for the generation of ROS by this route all along the membrane surface. O(2) reduction at the acceptor side of photosystem I remains poorly understood. The regeneration of antioxidants is stated to be a priority task of photosynthetic electron transport in view of the effectiveness of monodehydroascorbate as electron acceptor. We propose that ROS generation in the plastoquinone pool and the possible formation of hydroperoxides in the vicinity of photosystem II are key processes participating in the primary stages of redox signaling.