The aim of this study was to relate changes in the redox state of mitocondrial electron carriers to the 'burst' of oxyradicals in postischemic myocardium. The free radical EPR signals of control and re-oxygenated rat hearts were mainly due to coenzyme Q10, the line width was 0.81 +/- 0.02 mT, and the intensities (1.58 +/- 0.12) x 10(16) and (1.41 +/- 0.13) x 10(16) spins/g. The low-temperature spectra of oxygenated myocardium contained a predominant signal from a S3 Fe-S center and weak signals from N1b, N2, N3, N4 and S1 centers. Global ischemia caused cardinal changes in the redox state of the mitochondrial respiratory chain. The low-temperature EPR spectrum now contained intensive signals from most Fe-S centers. The amount of coenzyme Q10 semiquinones decreased during global ischemia, but the content of flavosemiquinones increased. The line width of the signal of the ischemic heart was 1.28 +/- 0.03 mT, and its intensity corresponded (3.16 +/- 0.94) x 10(16) spins/g. The spin-trapping experiments with TEMPONE-H showed that the rate of oxyradical generation in isolated cardiomyocytes essentially increased after hypoxia or on adding rotenone and antimycin A. It became equal to 4.2 +/- 0.3, 8.2 +/- 0.6 and 7.1 +/- 0.5 nmol/min mg-1 mitochondrial protein, respectively. The maximal stimulatory effect was observed in the presence of both inhibitors. The addition of superoxide dismutase, but not catalase, suppressed the formation of oxyradicals.