The effect of hyperoxia on activity of the superoxide-sensitive citric acid cycle enzyme aconitase was measured in cultured human epithelial-like A549 cells and in rat lungs. Rapid and progressive loss of > 80% of the aconitase activity in A549 cells was seen during a 24-hr exposure to a PO2 of 600 mmHg (1 mmHg = 133 Pa). Inhibition of mitochondrial respiratory capacity correlated with loss of aconitase activity in A549 cells exposed to hyperoxia, and this effect could be mimicked by fluoroacetate (or fluorocitrate), a metabolic poison of aconitase. Exposure of rats to an atmospheric PO2 of 760 mmHg or 635 mmHg for 24 hr caused respective 73% and 61% decreases in total lung aconitase activity. We propose that early inactivation of aconitase and inhibition of the energy-producing and biosynthetic reactions of the citric acid cycle contribute to the sequelae of lung damage and edema seen during exposure to hyperoxia.