The mitochondrial respiratory chain has been shown to produce H2O2 under certain conditions (Loschen, Flohe, and Chance, 1971; Boveris, Oshino, and Chance, 1972; Loschen, Azzui, and Flohe, 1973; Boveris and Chance, 1973). At least 1 percent of the total oxygen uptake in State 4 (Chance and Williams, 1956) in the presence of succinate is caused by H2O2 formation (Boveris et al., 1972). It was shown that the component responsible for mitochondrial H2O2 formation is located between the sites of the antimycin and rotenone inhibition (Loschen et al., 1971; Boveris et al., 1972). The possible physiological consequences of a respiratory chain-linked H2O2 formation is not yet understood. It would therefore be of significance to know whether oxygen radicals are involved in this process or not. It was recently suggested by Rotilio et al. (1973) that enzyme systems producing H2O2 via oxygen radicals are always closely associated with a superoxide dismutase. Thus, the existence of a mitochondrial superoxide dismutase located in the matrix space (Weisiger and Fridovich, 1973) prompted us to look for oxygen radicals as possible intermediates in the mechanism of the formation of mitochondrial H2O2. In the present communication we show that, under the same conditions in which H2O2 is formed in the mitochondrial respiratoy chain, oxygen radical formation can be demonstrated also. A possible relationship of this finding with lipid peroxidation and the toxic effects of hyperbaric oxygen is likely.