OBJECTIVE The aim was to determine if prostaglandin like activity might be involved in changes due to exercise training in the coronary flow responses to hypoxia. METHODS The coronary flow response to hypoxia was measured under constant perfusion pressure in isolated perfused hearts from 12 endurance exercise trained rats and 12 control rats. Eight hearts were perfused with a solution containing indomethacin, a cyclo-oxygenase inhibitor, to determine its effect on any training induced changes in the coronary flow response to hypoxic stress. METHODS 24 male Sprague-Dawley rats, 517 (SD 51) g, were used for this study. The animals were anesthetised and the hearts rapidly excised and perfused with a modified Langendorff perfusion system. RESULTS Under constant perfusion pressure, the hearts of endurance exercise trained rats had a greater increase in coronary flow during hypoxia relative to normoxia than did hearts of untrained rats, at 13.52(2.15) v 9.56(1.05) ml.min-1.g-1 dry heart weight. Indomethacin treatment abolished this difference and lowered coronary flow: exercise -3.81(3.75) ml.min-1.g-1; control 0.38(2.44) ml.min-1.g-1. CONCLUSIONS The inhibition by indomethacin of the endurance exercise training induced potentiation of the coronary fluid flow response to hypoxia suggests that prostacyclin or a related compound may be involved in this adaptation to exercise.