Although the presence of free radicals has been indicated in ischemic-reperfused heart, the exact nature and source of these free radicals are not known. The present study utilized a chemical trap, salicylic acid, to trap hydroxyl radical which could be detected as hydroxylated benzoic acid using high pressure liquid chromatography. Since the hydroxylated product is extremely stable, heart was subjected to subcellular fractionation after ischemia and reperfusion, and each fraction was separately examined for the presence of hydroxyl radical. The results indicated for the first time the presence of hydroxyl radical in the mitochondrial fraction during early reperfusion, which decreased in intensity as the reperfusion progressed.