1. Carbon monoxide (CO) acts competitively towards oxygen when the latter is taken up in respiration by cytochrome aa3-containing proteoliposomes, both in the presence of p-trifluoromethoxy carbonyl cyanide phenylhydrazone and valinomycin (deenergized state) and in their absence (energized state). At high levels of CO, the double reciprocal plots (1/v vs. 1/[O2]) in the energized and deenergized states are parallel, i.e. energization acts "anti-competitively" towards oxygen, and the "respiratory control ratio" decreases as the oxygen concentration decreases. 2. Azide acts non-competitively towards cytochrome c when the latter is oxidized by cytochrome aa3-containing proteoliposomes both in the energized and deenergized (plus p-trifluoromethoxy carbonyl cyanide phenylhydrazone and valinomycin) conditions. At low azide concentrations the apparent Ki for azide is unaffected by energization, but at high azide levels the Ki increases in energized liposomes, i.e. the "respiratory control ratio" decreases as the azide concentration increases. 3. It is concluded that the inhibitor experiments are consistent with but do not prove the concept that the oxidase molecules in a single vesicle are responding to a single "energization state" or set of electrochemical gradients. This and other models are discussed.