Free radicals have been implicated in several aspects of cellular injury, both during ischemia and reperfusion of the myocardium. In this study, formation of free radicals in the isolated rat heart was measured a) directly using electron paramagnetic resonance (EPR) spectroscopy and b) indirectly using the generation of thiobarbituric acid reactants as an index of lipid peroxidation. EPR spectra of frozen heart powder recorded at 100 degrees K show several lines and consist of different components separated by temperature studies: signal C disappears after warming the sample 1 minute at 190 degrees K and is suggestive of a triplet signal g = 2.001, aN = 25 Gauss; signal B g parallel = 2.034, g perpendicular = 2.007, disappears after 1 min at 240 degrees K, and is similar to those previously reported for oxygen alkylperoxyl free radical; the remaining signal, signal A with g = 2.004 is identical to that of a carbon-centered ubiquinone free radical. The total free radical concentration in isolated rat heart perfused at a constant flow rate of 12 ml/min was increased by 44% compared with control (p less than 0.05) after 10 minutes of normothermic global ischemia with a 10% residual flow, and by only 31% compared with control after 20 seconds of reflow with oxygenated perfusate (p less than 0.05). Compared with the reperfused group, trimetazidine 10(-5) M administered 15 minutes before the ischemic period decreased the free radical concentration (-20%). However, this free radical generation in heart was not associated with a concomitant increase of lipid peroxides.