Using principles developed by Kacser and Burns ((1973) in Rate Control of Biological Processes (Davies, D. D., ed) pp. 65-104, Cambridge University Press, London) and Heinrich and Rapoport ((1974) Eur. J. Biochem. 42, 97-105), inhibitor titration studies were carried out in order to quantify the amount of control (control strength) exerted by different steps in oxidative phosphorylation on the rate of mitochondrial oxygen uptake. In the resting state of respiration, nearly all control is exerted by the passive permeability of the mitochondrial inner membrane to protons. In the intermediate states and even in the active state of respiration, control is distributed among different steps including the adenine nucleotide translocator and cytochrome c oxidase. It is therefore not possible to speak of the rate-limiting step in oxidative phosphorylation. Since both the adenine nucleotide translocator and cytochrome c oxidase appear to be rate-controlling, hypotheses suggesting that respiration is fully controlled by either of these two steps alone need to be modified.