1. The electron flux through cytochrome oxidase is a linear function of the net thermodynamic force across the complex over a limited range of conditions. 2. Over a wide range of conditions the electron flux is a complicated function of the percentage reduction of the cytochrome c pool and of delta psi (at low values of delta pH). 3. We have estimated the elasticities of electron flux through cytochrome oxidase to delta Eh of the redox reaction catalysed by cytochrome oxidase (or to cyt c2+/cyt c3+) and to delta psi. The elasticities varied depending on the values of delta psi and of the percentage reduction of the cytochrome c pool. 4. At intermediate rates (which may correspond to those in vivo) the electron flux through cytochrome oxidase is controlled to about the same extent by delta psi and by delta Eh.