Flavoglaucin, a toxic metabolite from Eurotium (Aspergillus) chevalieri, has been found to induce a drastic swelling, to uncouple the oxidative phosphorylation, and to depress state 3 respiration in isolated rat liver mitochondria. It has been recently suggested that flavoglaucin depresses mitochondrial respiration by directly interacting with respiratory chain at complex III region in mitochondria. In the present study, it was confirmed that flavoglaucin indeed inhibited the reaction of the complex III at cytochromes bc₁, segment, accumulating a reduced form of cytochrome b, in the succinate oxidation system.