The respiratory chain of Rhodopseudomonas capsulata, strain St. Louis and of two respiration deficient mutants (M6 and M7) has been investigated by examining the redox and spectral characteristics of the cytochromes and their response to substrates and to specific respiratory inhibitors. Since the specific lesions of M6 and M7 have been localized on two different branches of the multiple oxidase system of the wild type strain, the capability for aerobic growth of these mutants can be considered as a proof of the physiological significance of both branched systems "in vivo". Using M6 and M7 mutants the response of the branched chain to respiratory inhibitors could be established. Cytochrome oxidase activity, a specific function of an high potential cytochrome b (E'0 = +413 mV) is sensitive to low concentrations of KCN (5-10(-5) M); CO is a specific inhibitor of an alternative oxidase, which is also inhibited by high concentrations of KCN (10(-3) M). Antimycin A inhibits preferentially the branch of the chain affected by low concentrations of cyanide. Redox titrations and spectral data indicate the presence in the membrane of three cytochromes of b type (E'0 = +413, +260, +47 vM) and two cytochromes of c type (E'0 = +342, +94 mV). A clear indication of the involvement in respiration of cytochrome b413, cytochrome c342 and cytochrome b47 has been obtained. Only 50% of the dithionite reducible cytochrome b can be reduced by respiratory substrates also in the presence of high concentrations of KCN or in anaerobiosis. The presence and function of quinones in the respiratory electron transport system has been clearly demonstrated. Quinones, which are reducible by NADH and succinate to about the same extent can be reoxidized through both branches of the respiratory chain, as shown by the response of their redox state to KCN. The possible site of the branching of the electron transport chain has been investigated comparing the per cent level of reduction of quinones and of cytochromes b and c as a function of KCN concentrations in membranes from wild type and M6 mutants cells. The site of the branching has been localized at the level of quinones-cytochrome b47. A tentative scheme of the respiratory chains operating in Rhodopseudomonas capsulata, St. Louis and in the two respiration deficient mutants, M6 and M7 is presented.