Mitochondria are exposed to large fluxes of iron, and reactive oxygen and nitrogen species. Hence they are susceptible to oxidative stress, a process inhibited by vitamin E. Our investigations show that iron uncouples oxidative phosphorylation whereas peroxynitrite and nitrite are inhibitors of oxidative phosphorylation. Oxidation of mitochondrial vitamin E is accompanied by generation of lipid peroxidation products, altered enzyme activity and electrical conductance etc., and result in inefficient oxidative phosphorylation. Vitamin E is important for mitochondrial function because: (1) Prior investigations have shown that vitamin E is essential for maintaining mitochondrial respiration. (2) Vitamin E is the most potent, lipid-soluble antioxidant localized ideally in mitochondrial membranes. (3) The decline in respiratory control ratios (RCR) of rat brain mitochondria exposed to peroxynitrite closely paralleled the oxidative elimination of vitamin E. (4) Finally, iron is a strong uncoupler of oxidative phosphorylation in brain mitochondria from vitamin E deficient animals and not from controls.