The principle mitochondrial target where the respiratory inhibitors CO, CN(-), and NO act in the execution of their acute toxic effects is complex IV of the electron-transport chain, cytochrome c oxidase. However, there is a paucity of studies in the literature regarding the concerted effects of such poisons. Accordingly, the combined inhibitory effects of CO + CN(-), NO + CN(-), and NO + CO on the activity of cytochrome c oxidase preparations are reported. Only in the case of CO + CN(-) do the effects of the two inhibitors seem to be additive as expected. NO appears to be antagonistic toward the effects of the other two inhibitors; that is, the effects of both CO an CN(-) on enzyme activity are ameliorated by NO when present. To further clarify these observations, the ligand substitutions of heme-bound CN(-) by NO in cytochrome c oxidase and hemoglobin have also been briefly investigated. These results suggest that displacement of CN(-) from the ferric hemoproteins by NO is rate-limited by heme reduction-and in the case of the enzyme, the presence of nonligand-binding electron-transfer centers facilitates the reaction. The findings are discussed in relation to the idea that NO does not behave as a classic reversible (by dissociation) inhibitor.