The reactions of nitric oxide (NO) with both oxidized and reduced cytochrome c oxidase are reported. NMR and mass spectroscopy were utilized to determine the products of the reactions; EPR and optical spectroscopy were employed to determine the states of the enzyme produced in each of these reactions. It was found that the enzyme catalyzes the consecutive oxidation and reduction of NO. A different cycle was observed when NO was added to the reduced enzyme, to the oxidized enzyme, or to the oxidized enzyme in the presence of azide. It was possible to observe the state of the enzyme at several points in each of these three cycles by varying the concentration of NO. The reactions of NO all involved a one- or two-electron redox step and could be accounted for by the involvement of only cytochrome a3 and Cua3. On the basis of these results, a mechanism for the reduction of dioxygen by the enzyme is proposed in which cytochrome a3 functions to anchor dioxygen and intermediates while remaining in the ferrous state, whereas Cua3 functions to accept electroins from cytochrome a/Cua and transfer them to dioxygen.