The rate of CO 2-and p-benzoquinonedependent photosynthetic O2 evolution by Anabaena variabilis cells remained unaltered and the rate of O2 uptake observed after switching off the light (endogenous respiration) was enhanced by a factor of 6-8 when the O2 concentration was increased from 200 to 400 gM. Photosystem-I-linked 02 uptake and respiration of the cells incubated with ascorbate and N,N,N'N'-tetramethyl-p-phenylenediamine was not appreciably influenced by the O2 concentration. 2-Iodo-6-isopropyl-3methyl-2',4,4'-trinitrodiphenyl ether, blocking electron transfer at the plastoquinone level, suppressed 02 evolution and had no influence on endogenous respiration. 2-n-Heptyl-4-hydroxyquinoline-N-oxide, an inhibitor of electron transfer between photosystems II and I, as well as the cytochrome-oxidase inhibitors N~, CN-and NH2OH, caused a 35-50% retardation of endogenous respiration and blocked photosynthetic O2 evolution. The molar ratio of cytochromes b6, f, c-553, aa3 and photosystem-I reaction centers in the isolated membranes equalled approx. 2: 1 : 2: 0.7: 2. It is inferred that endogenous respiration of A. variabilis cells is inhibited by the light-induced electron flow through both photosystems at the level of the plastoquinone-plastocyanin-oxidoreductase complex.