This study examines the possible role of Coenzyme Q (CoQ, ubiquinone) in the control of mitochondrial electron transfer. The CoQ concentration in mitochondria from different tissues was investigated by HPLC. By analyzing the rates of electron transfer as a function of total CoQ concentration, it was calculated that, at physiological CoQ concentration NADH cytochrome c reductase activity is not saturated. Values for theoretical Vmax could not be reached experimentally for NADH oxidation, because of the limited miscibility of CoQ10 with the phospholipids. On the other hand, it was found that CoQ3 could stimulate alpha-glycerophosphate cytochrome c reductase over three-fold. Electron transfer being a diffusion-coupled process, we have investigated the possibility of its being subjected to diffusion control. A reconstruction study of Complex I and Complex III in liposomes showed that NADH cytochrome c reductase was not affected by changing the average distance between complexes by varying the protein: lipid ratios. The results of a broad investigation on ubiquinol cytochrome c reductase in bovine heart submitochondrial particles indicated that the enzymic rate is not diffusion-controlled by ubiquinol, whereas the interaction of cytochrome c with the enzyme is clearly diffusion-limited.