The redox state of the carriers of electron-transport chain of cardiac mitochondria was studied in the conditions of normal perfusion, global ischemia and reoxygenation of the myocardial tissue. Experiments were performed on isolated rat hearts perfused at 37 degrees C by the "working heart" procedure. The EPR spectra of the freeze-clamped hearts were measured at 6-30 K. An analysis of the main values of g-tensor, line-shape, line-width and relaxation parameters of the components of low-temperature EPR spectra allowed to distinguish the signals from Fe-S centers of NADH-CoQ reductase and succinate-CoQ reductase, and the signals from free radical species of coenzyme Q and flavin coenzymes. The EPR spectra of hearts that were fixed during control perfusion and reperfusion contained predominantly the signal of oxidized S3 center of succinate-CoQ reductase. The free radical signal in these conditions was mainly due to ubisemiquinones. Besides the intensive signal of S3 center, the low-temperature EPR spectra contained also signals from different Fe-S centers paramagnetic in reduced state. The global ischemia of cardiac muscle caused essential reduction of the Fe-S clusters of the mitochondrial electron-transport chain. In ischemic condition the free radical EPR signal was mainly due to flavosemiquinones. The changes of the redox state of carriers of the mitochondrial respiratory chain correlated with the changes of the physiological parameters of cardiac muscle.