Vascular responses to endogenous agonists may determine patency rates of bypass graft conduits. The effect of constrictors (noradrenaline, phenylephrine, serotonin, histamine, angiotensin II) and dilators (acetylcholine, substance P, bradykinin, nitroglycerin) were compared in human internal mammary and inferior epigastric arteries in vitro. The latter vessel type has been recently advocated as an additional conduit for coronary artery bypass grafting. Whereas the alpha-adrenoceptor- (noradrenaline, phenylephrine) and serotonin receptor-mediated contractions were similar in both vessels, histamine-induced contractions were greatly enhanced in internal mammary arteries (maximal responses in percent of 80 mmol/L KCl, 131% +/- 15% versus 59% +/- 8%). Maximal contractions in response to angiotensin II were greater in inferior epigastric arteries (50% +/- 6% versus 25% +/- 5%). The endothelium-independent relaxations in response to nitroglycerin were identical in both vessels. In contrast, the endothelium-dependent relaxations in response to acetylcholine, substance P, and bradykinin were significantly greater in the inferior epigastric than in the internal mammary arteries (maximal relaxations expressed as percent of prostaglandin F2 alpha-induced precontraction: acetylcholine, 94% +/- 5% versus 77% +/- 5%; substance P, 85% +/- 4% versus 24% +/- 5%; bradykinin, 77% +/- 5% versus 26% +/- 3%). It is concluded that the inferior epigastric artery has a high endothelial capacity to release endothelium-derived relaxing factor. It appears that the inferior epigastric artery possesses credentials to be successfully used for coronary artery bypass grafting.