In isolated perfused rat hearts coronary flow appears to be regulated through the tissue energy level ([ATP]free/[ADP]free[Pi]), a metabolic product of oxygen metabolism by mitochondria, and not by the tissue oxygen tension. Mitochondrial oxidative phosphorylation in intact cells and tissues is dependent on oxygen tension throughout the physiological range. This allows oxidative phosphorylation to serve as an important tissue oxygen sensor. The apparent Km of mitochondrial cytochrome c oxidase for oxygen is dependent on the pH of the suspending medium, [ATP]/[ADP][Pi] and the state of reduction of cytochrome c. When physiological levels of [ATP]/[ADP][Pi], pH and cytochrome c reduction are used, oxidative phosphorylation by suspensions of isolated mitochondria has an oxygen dependence similar to that observed in intact cells.