Membranes isolated from Nostoc sp. strain MAC and Anacystis nidulans displayed spectral changes in the cytochrome f b region when examined by reduced minus oxidized or dual wavelength spectrophotometry under physiological conditions. The same changes accompanied both light-induced (photosynthetic) and oxygen-induced (respiratory) electron transport. Physiological reduction of the cytochrome f b moiety was abolished after extraction of plastoquinone but reappeared on reconstitution of the depleted membranes with authentic plastoquinone. Moreover, a mutual inhibition of photosynthetic and respiratory activities could be directly demonstrated with the isolated membranes. From the results it is concluded that the membrane-bound plastoquinol-cytochrome f b reductase functions as a common electron donor to both P700 and the cytochrome oxidase in cyanobacteria.