Abstract: The formation of hydroxyl radical in the extracellular fluid in striatum of rats subjected to 4-vessel occlusion during ischemia and reperfusion was measured by HPLC with UV detector using salicylic acid as a trapping agent. Hydroxyl radicals react with salicylic acid yielding 2,3-and 2, 5-dihydroxybenzoic acids which can be separated by liquid chromatography. The striatum was perfused with artificial cerebrospinal fluid containing 0.5 mmol·L^-1 salicylic acid at a flow rate of 2.5 μl·min^-1. The results indicate that the concentration of 2, 5-dihydroxybenzoic acid in the perfusates was highest during 25 min reperfusion and no change was found during ischemia. Vit E at 30,60 and 120 mg·kg^-1 i.p. 20 min before ischemia was shown to dose dependently suppress the rise of 2,5-dihydroxybenzoic acid after reperfusion. The results suggest that the HPLC method is useful and reliable for the measurement of hydroxy radical in global ischemia in vivo.