1. 1. The inhibition of rat-liver mitochondrial respiration in the presence of a-ketoglutarate, glutamate, succinate and β-hydroxybutyrate as a substrates by the murine toxin of Pasteurella pestis is not relieved by 2,4-dinitrophenol indicating that the toxin exerts its inhibitory effect somewhere in the electron-transport chain. 2. 2. Absolute and difference spectra of the cytochrome components of toxin-treated rat-heart and -liver mitochondria reveal that toxin addition causes on oxidation of cytochromes a, b and c suggesting that the toxin acts between reduced nicotinamide-adenine dinucleotide or succinate and cytochrome b. 3. 3. The oxidation of ascorbate by rat and liver mitochondria in the presence of tetramethylphenylenediamine and cytochrome c as carriers is not affected by the toxin. 4. 4. The reduction of cytochrome c in the presence of reduced nicotinamide-adenine dinucleotide by rat-heart mitochondria and electron-transport particles is inhibited by the toxin.