Although a burst of oxidants has been well described with reperfusion, less is known about the oxidants generated by the highly reduced redox state and low O₂ of ischemia. This study aimed to further identify the species and source of these oxidants. Cardiomyocytes were exposed to 1 h of simulated ischemia while oxidant generation was assessed by intracellular dihydroethidine (DHE) oxidation. Ischemia increased DHE oxidation significantly (0.7 ± 0.1 to 2.3 ± 0.3) after 1 h. Myxothiazol (mitochondrialsite III inhibitor) attenuated oxidation to 1.3 ± 0.1, as did the site I inhibitors rotenone (1.0 ± 0.1), amytal (1.1 ± 0.1), and the flavoprotein oxidase inhibitor diphenyleneiodonium (0.9 ± 0.1). By contrast, the site IV inhibitor cyanide, as well as inhibitors of xanthine oxidase (allopurinol), nitric oxide synthase (nitro-l-arginine methyl ester), and NADPH oxidase (apocynin), had no effect. Finally, DHE oxidation increased with Cu- and Zn-containing superoxide dismutase (SOD) inhibition using diethyldithiocarbamate (2.7 ± 0.1) and decreased with exogenous SOD (1.1 ± 0.1). We conclude that significant superoxide generation occurs during ischemia before reperfusion from the ubisemiquinone site of the mitochondrial electron transport chain.