In a model of autoimmune myocarditis in guinea piga, the authors studied energy producing processes in mitochondria and the role of oxygen free radical generation in tissue injury. A significant decrease in the capacity of oxidative phosphorylation was detected in parameters QO2 (S3) on the basis of glutamate + malate (p < 0.005) and pyruvate (p < 0.05) measurements. Other parameters studied were however only insignificantly reduced in myocarditic heart mitochondria as compared to controls. The activity of mitochondrial cytochrome oxidase was also insignificantly reduced in animals with myocarditis. Generation of oxygen free radicals was in the presence of the inflammatory infiltrate in this model not significantly higher than in the intact myocardial tissue.