This study tested the hypothesis that metabolites of arachidonic acid may contribute to the coronary vascular response to physiological increases of myocardial O2 consumption that occur during exercise. Studies were performed in eight chronically instrumented dogs with electromagnetic flowmeter probes on the left circumflex coronary artery and aortic and coronary sinus catheters. Data were obtained at rest and during graded treadmill exercise during control conditions and after administration of the cyclooxygenase inhibitor indomethacin. During control conditions heart rate, aortic pressure, and coronary blood flow increased progressively during exercise; this was accompanied by a significant increase in myocardial O2 extraction, as evidenced by a decrease in coronary venous O2 tension (Po2) particularly during the first stage of exercise. Indomethacin (5 mg/kg iv) resulted in marked blunting of the coronary vasodilator response to intracoronary arachidonic acid in anesthetized open-chest dogs. After administration of indomethacin to awake dogs, resting heart rate, aortic pressure, and coronary venous Po2 were unaltered, and the response of these variables to exercise was not changed. The increase in coronary blood flow during exercise was also unchanged after indomethacin, so that the relationship between myocardial O2 consumption and coronary blood flow was unaltered by cyclooxygenase inhibition. Thus we were unable to demonstrate a significant effect of the prostaglandin system in mediating the coronary vascular response to exercise.