To elucidate the pathophysiological role of the hydroxyl radical (OH) during the postischemic reperfusion of the heart, we measured the OH product in the coronary effluent from isolated perfused rat heart during a 30-minute reperfusion period after various ischemic intervals of 5, 10, 15, 20, 30, and 60 minutes. Salicylic acid was used as the probe for OH, and its derivative, 2,5-dihydroxybenzoic acid (2,5-DHBA), was quantified using high-performance liquid chromatography with ultraviolet detection. 2,5-DHBA was negligible in the effluent from nonischemic hearts, but a significant amount was detected from the hearts rendered ischemic for 10 minutes or longer. The peak of 2,5-DHBA was seen within 90 seconds after the onset of reperfusion in every group. The accumulated amount of 2,5-DHBA was maximal in the group with 15-minute ischemia (6.73± 1.04 nmol/g wet heart wt after 30 minutes of reperfusion); it decreased as the ischemic time was prolonged and was 2.38±0.84 nmol/g wet wt after 30 minutes of reperfusion in the group with 60-minute ischemia. In the model of 15-minute ischemia/30-minute reperfusion, there was no correlation between the accumulated amount of 2,5-DHBA and functional recovery (±dP/dt, heart rate, and coronary flow), lactate dehydrogenase release, and morphological damage. Although treatment with 0.5 mM deferoxamine, an iron chelator, significantly decreased 2,5-DHBA (from 6.73±1.04 to 2.29±0.80 nmol/g wet wt after 30 minutes of reperfusion,p<0.01), it failed to reduce the postischemic myocardial injury in the group with 15-minute ischemia. The results suggest that OH production is influenced by the preceding ischemic interval and that OH does not exert an immediate direct effect on postischemic damage during early reperfusion in the isolated perfused rat heart, although a possibility remains that the small portion of OH trapped by salicylic acid may not be intimately associated with myocardial injury.