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he "oxyradical hypothesis" of myocardial stunning proposes that superoxide, released on reperfusion, leads to contractile dysfunction via the production of the more reactive hydroxyl free radical from the iron catalysed Haber-Weiss reaction.However, superoxide reacts many times faster with nitric oxide (NO), than with ferric iron, leading to the formation of peroxynitrite (ONOO -), which is a potent oxidising and nitrating agent with properties and reactivity similar to hydroxyl radicals.Consequently ONOO -is a candidate for the initiation of oxidation reactions following brief ischaemia-reperfusion injury.The production of reactive nitrogen intermediates such as ONOO -is reflected in vivo by the formation of the amino acid derivative 3-nitrotyrosine.Recently, we have developed a sensitive method for the quantitative analysis of tissue nitrotyrosine which avoids the artefactual formation of nitrotyrosine that can occur with more conventional protein preparative steps. 1 The present study sought to test the hypothesis that myocardial stunning causes increased formation of tissue nitrotyrosine.. . .