Specific features of myocardial damage induced by overload and the very action of reactive oxygen species were analyzed and characterized. The experiments were performed on the isolated perfused heart of the rat (according to Langendorff). Overload of the heart by calcium ions was studied on the model of calcium paradox; damage of the heart by reactive oxygen species was induced by a single application of a free radicals generating system. Functional, biochemical, and ultrastructural parameters were studied. Several of the examined parameters (lactate accumulation, drop of total adenine nucleotides, transmural heterogeneity in the histochemical reactivity of enzymes) are of a certain value in recognizing the participation of individual factors involved in reperfusion induced damage of the myocardium.