Responses of human and canine washed platelets to thrombin in the presence and absence of human and canine coronary arteries were examined to establish a simple indirect measurement system for endothelium-derived relaxing factor (EDRF). Isolated rings of coronary arteries were obtained from either adult dogs or recipient hearts of cardiac transplant patients. The addition of small segments of canine and human coronary arteries with intact endothelia inhibited thrombin-induced platelet aggregation, but those with disrupted endothelia did not. Pretreatment of human and canine coronary arteries with the cyclo-oxygenase inhibitor indomethacin markedly attenuated the observed effect. The anti-platelet aggregatory effect of indomethacin-pretreated coronary artery was enhanced by acetylcholine or histamine, stimulators of EDRF production, or superoxide dismutase, which prolongs the half-life of EDRF, and inhibited by the EDRF inhibitor hemoglobin. These results suggest that endothelial cells of the human coronary artery, like the canine coronary artery, can produce EDRF which inhibits platelet aggregation, and that this simple experimental model is useful to examine the effect of environmental chemicals on EDRF in the human coronary artery.