—Oxidative stress in the myocardium may play an important role in the pathogenesis of congestive heart failure (HF). However, the cellular sources and mechanisms for the enhanced generation of reactive oxygen species (ROS) in the failing myocardium remain unknown. The amount of thiobarbituric acid reactive substances increased in the canine HF hearts subjected to rapid ventricular pacing for 4 weeks, and immunohistochemical staining of 4-hydroxy-2-nonenal ROS-induced lipid peroxides was detected in cardiac myocytes but not in interstitial cells of HF animals. The generation of superoxide anion was directly assessed in the submitochondrial fractions by use of electron spin resonance spectroscopy with spin trapping agent, 5,5′-dimethyl-1-pyrroline-N-oxide, in the presence of NADH and succinate as a substrate for NADH–ubiquinone oxidoreductase (complex I) and succinate–ubiquinone oxidoreductase (complex II), respectively. Superoxide production was increased 2.8-fold (P<0.01) in HF, which was due to the functional block of electron transport at complex I. The enzymatic activity of complex I decreased in HF (274±13 versus 136±9 nmol · min⁻¹ · mg⁻¹ protein, P<0.01), which may thus have caused the functional uncoupling of the respiratory chain and the deleterious ROS production in HF mitochondria. The present study provided direct evidence for the involvement of ROS in the mitochondrial origin of HF myocytes, which might be responsible for both contractile dysfunction and structural damage to the myocardium.