OBJECTIVE There is experimental evidence that oxygen-derived free radicals (Superoxide (O(2)(-))) play a key role in tissue damage in ischemia-reperfusion injury. Among various antioxidants tested in vitro, natural polyphenols like Epigallocatechine gallate (EGCG) show a 164-fold higher scavenging activity for O(2)(-) than ascorbic acid We therefore conducted an animal study in order to investigate the impact of EGCG on O(2)(-) production during reperfusion after defined periods of ischemia in the muscle tissue of the rat, using a recently developed cytochrome c-based biosensor for on-line in vivo monitoring of O(2)(-). METHODS Femoral artery and vein were dissected below the inguinal ligament in male Wistar rats. The cytochrome c-based biosensor was placed in the gastrocnemius muscle. Ischemia was induced by clamping the femoral vessels. Ischemia times were either 60 (n = 14) or 120 (n = 14) minutes. Six animals in each group received 4 mg/kg body weight EGCG intravenously at the time of reperfusion, another six animals in each group served as controls (no treatment). Additionally, two animals in each group received the same volume of saline instead of EGCG. The current response of the biosensor corresponding to the O(2)(-) concentrations in vivo was recorded on a PC. The gastrocnemius muscles were harvested for histological evaluation. RESULTS The average maximum O(2)(-) concentration after 60 minutes of ischemia was 188, 18 nmol/L (23 pA) compared to 90 nmol/L (11 pA) (P <.01) with EGCG application. The mean O(2)(-) value after 120 minutes was 220 nmol/L (27 pA) versus 135 nmol/L (16.5 pA) (P <.01) with EGCG, respectively. Histological analysis showed advanced muscle cell injury and neutrophil infiltration in the group without EGCG. No O(2)(-) reduction could be verified administering saline instead of EGCG. CONCLUSIONS For the first time the scavenging activity of an antioxidant was verified in vivo on-line. EGCG significantly diminished O(2)(-) tissue concentrations after 60 or 120 minutes of ischemia by an average of nearly 50%, suggesting its therapeutic potential.