Elongation factor G (EF-G), a key protein in translational elongation, is inactivated by reactive oxygen species via formation of an intramolecular disulfide bond between two specific cysteine residues in Synechocystis sp. PCC 6803. The oxidized EF-G can be reduced and reactivated by thioredoxin. However, it remains to be clarified whether EF-G is reduced by reducing equivalents derived from the photosynthetic transport of electrons in vivo. We attempted to establish method by which the redox state of EF-G can be monitored in vivo. After Synechocystis cells were exposed to light at various intensities, cells were supplemented with methanol to fix the redox state of proteins. Cells were broken and SH-groups of proteins were modified with PEG-maleimide, and then the reduced and oxidized forms of EF-G were detected immunologically. When the intensity of light was raised, the ratio of reduced form of EF-G increased. However, under strong light, the ratio of oxidized form increased. These observations suggest that EF-G might be regulated by reducing equivalents that are generated by the photosynthetic transport of electrons and mediated by thioredoxin.