The redox reactions of the bis-heme cytochrome b of the ubiquinol:cytochrome c oxidoreductase complex (complex III, bc1 complex) were studied in bovine heart submitochondrial particles (SMP). It was shown that (i) when SMP were treated with the complex III inhibitor myxothiazol (or MOA-stilbene or stigmatellin) or with KCN and ascorbate to reduce the high potential centers of complex III (iron-sulfur protein and cytochromes c + c1), NADH or succinate reduced heme bL slowly and incompletely. In contrast, heme bH was reduced by these substrates completely and much more rapidly. Only when the complex III inhibitor was antimycin, and the high potential centers were in the oxidized state, NADH or succinate was able to reduce both bH and bL rapidly and completely. (ii) When NADH or succinate was added to SMP inhibited at complex III by antimycin and energized by ATP, the bis-heme cytochrome b was reduced only partially. Prereduction of the high potential centers was not necessary for this partial b reduction, but slowed down the reduction rate. Deenergization of SMP by uncoupling (or addition of oligomycin to inhibit ATP hydrolysis) resulted in further b reduction. Addition of ATP after b was reduced by substrate resulted in partial b oxidation, and the heme remaining reduced appeared to be mainly bL. Other experiments suggested that the redox changes of cytochrome b effected by energization and deenergization of SMP occurred via electronic communication with the ubiquinone pool. These results have been discussed in relation to current concepts regarding the mechanism of electron transfer by complex III.