The electron-transfer activities of bovine heart rnitochondrial complexes I, II, and III, but not complex IV, were simultaneously inhibited by 2-alkyl-4, 6-dinitrophenols to a different extent. The extent of inhibition of NADH and succinate oxidase activities by dinitrophenols was compared with that of individual complex activities using submitochondrial particles. The extent of inhibition of succinate oxidase activity by 1-methyipropyl and 1-methylbutyl derivatives was much larger than that of NADH oxidase activity. This large inhibition of succinate oxidase activity seemed not to be explainable by the extent of inhibition of individual complex activities (i.e., complexes II and III activities), based upon the homogeneous ubiquinone pool model. On the other hand, other dinitrophenols (n-propyl, 1-methylpentyl, 1-methylhexyl, and tert-butyl derivatives) very similar to the above compounds did not elicit such anomalous inhibitory action, indicating that the action of 1-methylpropyl and 1-methylbutyl derivatives is highly specific to their structure. The anomalous inhibition by these two compounds was also observed with the isolated succinate-cytochrome c oxidoreductase, in which there is no ubiquinone pool behavior [Rich, P. R. (1984) Biochim. Biophys. Acta 768, 53-79]. However, when the succinate-cytochrome c reductase of which the activity had been partially restored by adding phospholipid and exogenous quinone to the phospholipid- and ubiquinone-depleted succinate-cytochrome c reductase was assayed, the anomalous inhibitory action of interest was undetectable. These results indicated that electron-transfer between complexes II and III, which is mediated not only by free-form, but also by protein-bound ubiquinone, occurs in the mitochondrial membrane. The fact that the anomalous inhibition of succinate oxidase activity of submitochondrial particles was sensitive to changes in the external osmotic pressure which affected the total area of the particle supports this notion.