In the last few years much attention has been dedicated to the elucidation of some of the molecular aspects of cytochrome c oxidase. It has been shown conclusively that the enzyme from several sources (yeast, Neurospora, heart, liver) contains seven different subunits, which are asymmetrically inserted in the membrane. All of these are in contact with the lipid bilayer (except subunits V and VI) and to a greater or lesser extent with the water phase as well (except for subunit I). Subunit II of the enzyme appears to be involved in the formation of the binding site of cytochrome c. The location of the redox groups of the enzyme is still a matter of controversy. Their distance from the cytochrome c heme group is approximately 35 A such that electron tunneling appears to be the only possible mechanism for transporting electrons across such a distance. A proton pump appears to be associated with electron transport and approximately one proton is extruded per electron equivalent reducing oxygen via the enzyme. N,N', dicyclohexylcarbodiimide a well-established inhibitor of H+-translocating ATPases inhibits the proton pump and labels specifically subunit III of the enzyme.