Abstract 1. The inhibitory and spectroscopic effects of azide on phosphorylating and non-phosphorylating systems have been studied. In non-phosphorylating particles succinate oxidase inhibition requires 10 times the concentration of azide needed to inhibit the oxidase itself. 2. Reduction of cytochrome a in non-phosphorylating particles oxidizing succinate occurs at low azide concentrations; inhibition supervenes only if sufficient azide is present to cause appreciable steady-state reduction of endogenous cytochrome c. 3. The previously reported shift in the α band of reduced cytochrome a from 605 to 600 mμ in presence of azide is now observed in all respiratory states of the mitochondrion, in Keilin-Hartree particles and in the isolated oxidase. It is concluded that this form of cytochrome a is neither a high-energy compound nor a complex with azide but the result of heme-heme interaction between reduced a and the classical oxidized a 3-azide complex. 4. The high sensitivity of mitochondrial respiration in ADP-stimulated State 3 to azide is not accompanied by an increased sensitivity of mitochondrial oxidase activity. The changes in steady-state reduction of cytochromes c and a parallel the inhibition of respiration. Relief of inhibition by uncouplers is not explained by changes in intramitochondrial azide concentration and does not involve the disappearance of cytochrome a with an α band at 600 mμ. 5. It is suggested that the relief of azide inhibition by uncouplers is due to a change in electron-transfer pathways permitting a direct reduction of a 3 by c and short-circuiting cytochrome a. 6. The spectrum of reduced a 3 is discussed and it is indicated that this cytochrome probably has no appreciable α band at 605 mμ but instead is characterized in the visible region solely by a broad band centered at 560 mμ.