Oxidation of the respiratory chain carriers of anaerobic, CO-saturated skunk cabbage (Symplocarpus foetidus) mitochondria, by means of an O(2) pulse, proceeds primarily through the cyanide-insensitive alternate oxidase, since the oxidation of cytochromes a and a(3) takes place with a half-time of 3 seconds, corresponding to the rate of dissociation of CO from reduced cytochrome a(3). Ubiquinone and part of the flavoprotein are oxidized within 1 second under these conditions, and this rapid rate of oxidation is strongly inhibited by m-chlorobenzhydroxamic acid (mCLAM), a specific inhibitor of the alternate oxidase of plant mitochondria. The rate of ubiquinone oxidation under these conditions in white potato (Solanum tuberosum) mitochondria, which have no alternate oxidase, is the same as that in skunk cabbage mitochondria treated with mCLAM. Ubiquinone, thus identified as the carrier common to both the cytochrome and alternate oxidase pathways, is linked to the alternate oxidase by a flavoprotein of midpoint potential 50 millivolts more negative with which it is in equilibrium. This arrangement provides a switch for diverting electron transport primarily through the cytochrome pathway under state 3 conditions and primarily through the alternate oxidase pathway under state 4 conditions.