Abstract 1. 1. The functions of ubiquinone in the respiratory chain were studied with membrane fractions from aerobically grown Rhodopseudomonas palustris. 2. 2. Ubiquinone-10 was found to be the only quinone present (5.70 nmoles/mg protein). 3. 3. The extent of reduction of ubiquinone was independent of the respiratory rates of the substrates. 80% of the total amount of ubiquinone was equally accessible to the redox equivalents provided by NADH and succinate. 4. 4. Extraction of the membranes with n-pentane, or irradiation with ultraviolet light, greatly lowered the activities of both the NADH and succinate oxidase systems. On reincorporation of ubiquinone into the ubiquinone-depleted membranes, both the NADH and succinate oxidase systems were restored. The restoration of the succinate oxidase system was found to be independent of the length of the isoprenoid side chain of ubiquinone, whereas only the higher homologues of ubiquinone were active in the NADH oxidase system. Phospholipid was essential for the effect of ubiquinone homologues in restoring the NADH oxidase system, whereas no effect was found for restoration of the succinate oxidase system. 5. 5. The ubiquinone-depleted membranes exhibited no, or very small, cytochrome reduction with NADH or succinate. The cytochromes of normal, lyophilized, and ubiquinone-incorporated membranes were all equally reduced by succinate, NADH and dithionite. 6. 6. Neither extraction nor irradiation of endogenous ubiquinone had any effect on the NADH dehydrogenase of the membranes as measured with 2,6-dichlorophenolindophenol as the electron acceptor, whereas the succinate dehydrogenase activity measured with phenazine methosulphate-2,6-dichlorophenolindophenol as the electron acceptor was diminished. However, this latter activity was fully restored when ubiquinone was reincorporated into the membranes. 7. 7. It is concluded that ubiquinone not only serves as a redox carrier between the dehydrogenases of NADH and succinate, respectively, and the cytochromes, but may also exert an effect on the succinate dehydrogenase. It was further concluded that the sites for ubiquinone are structurally different in the two oxidase systems. The site for ubiquinone in the NADH oxidase system has, in contrast to the site in the succinate oxidase system, a specific requirement for the high hydrophobicity of an isoprenoid side chain and for phospholipid. However, these two sites interact with each other and exchange electrons.