We have studied the reactions of the oxidase of Paracoccus dentrificans with its membrane-bound cytochrome c and with soluble cytochrome c550 of Paracoccus and of bovine heart. The turnover rate of Paracoccus oxidase with membrane-bound cytochrome c is high, approaching 1000/sec. at 25 degrees. When soluble cytochrome c is added to the electron transport chain oxidizing NADH or succinate, no increase in 02 uptake is observed. When the oxidase is reacting with the membrane-bound cytochrome c, the reaction site is not exposed for reaction with soluble cytochrome c. We have purified the Paracoccus oxidase, following relatively simple methodology. It has three major subunits similar in molecular weight to those of the larger subunits of the bovine oxidase. When reconstituted in the presence of asolectin, it is just as active as the intact membrane-bound oxidase in reaction with soluble cytochrome c. The soluble cytochrome c reacts directly with the cytochrome aa3. We found direct evidence that the oxidase is stimulated in the presence of low concentrations of cytochrome c. The stimulatory effect could be the explanation for the so-called "high affinity" site for reaction with cytochrome c. The reaction of bovine cytochrome c with Paracoccus oxidase resembles that with the bovine oxidase in every way tested. The Paracoccus oxidase must have a cytochrome c binding site equivalent to that of the bovine enzyme. The reaction of the Paracoccus oxidase with its own soluble cytochrome c550, which has a highly negative hemisphere on the side of the molecule away from the heme crevice, has different properties from those seen in its reaction with bovine cytochrome c. However the properties all change to be like those with bovine cytochrome c on addition of poly-L-lysine. These data emphasize the importance of all of the charged groups on the cytochrome c in influencing binding or electron transfer reactions. The respiratory chain on the membranes of a cytochrome c-deficient mutant can reduce cytochrome aa3 using NADH as substrate in a manner similar to that of the wild type, although at somewhat lower rate, suggesting diffusional encounter of the large complexes within the membrane. Our data permit speculations about the possible evolution from the bacterial to the mitochondrial electron transport system.