Background The gastroepiploic artery is increasingly used as an alternative arterial coronary bypass conduit.In vitro studies have reported differences in vasoreactivity among various types of coronary graft conduits, susceptible to influencing the adaptation of myocardial blood flow and long-term patency rate.Methods and Results To evaluate in vivo the vasoreactivity of gastroepiploic artery grafts implanted long-term, nine angiographically smooth grafts implanted to the distal right or to the left circumflex coronary artery were studied with quantitative angiography 6 to 36 months after surgery. Angiograms were obtained on 35mm cinefilms in basal conditions, after injection of methylergometrine (0.4 mg IV), and after intragraft injection of 1 mg isosorbide dinitrate. In basal conditions, there was no difference in luminal diameter between gastroepiploic and coronary arteries (1.64+-0.32 versus 1.51+-0.31 mm; P=NS). After methylergometrine, a constriction was observed in all gastroepiploic artery grafts (-14+-6% of basal diameter) and in all but one grafted coronary artery (-6+-5%). After isosorbide dinitrate, a dilation was consistently observed in all gastroepiploic artery grafts (+26+-9%) and grafted coronary arteries (+14+-7% of basal). Changes in lumen diameter in response to these constrictor and dilator stimuli, either expressed in absolute values or in percentage of control were significantly greater (P<.001) in gastroepiploic artery grafts than in grafted coronary arteries.Conclusions Gastroepiploic artery grafts implanted long-term are more reactive than grafted coronary arteries to ergometrine and nitrates. This response differs from that previously reported of internal mammary artery grafts to the same pharmacological vasoactive stimuli. This suggests that the concept of a more efficient endothelium-dependent control of vasomotor tone contributing to better long-term functional results of internal mammary artery grafts cannot be directly extrapolated to gastroepiploic artery grafts. (Circulation. 1994;90(part 2):II-155-II-159.)