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BACKGROUND. In vitro studies have shown that the vasomotor response of internal mammary artery to vasoactive agents differs from that of saphenous vein. METHODS AND RESULTS. To assess whether the response in vivo to methylergometrine and to nitrates differs in saphenous vein grafts, internal mammary artery grafts, and coronary arteries, 25 patients were studied more than 6 months (range, 6-96 months) after surgery. Angiograms of saphenous vein grafts (n = 11) or mammary artery grafts (n = 14) were obtained in basal conditions, after intravenous infusion of 0.4 mg methylergometrine, and after intragraft infusion of 1 mg isosorbide dinitrate. Computerized quantitative angiography was used to assess the changes in luminal diameter of the bypass graft and of the grafted coronary artery (n = 11). Methylergometrine reduced the diameter of saphenous vein grafts by 6.9 +/- 7.4%, from 3.26 +/- 0.71 to 3.05 +/- 0.76 mm (p < 0.01), and that of grafted coronary arteries by 9.3 +/- 7.2% (NS compared with saphenous vein grafts), from 2.08 +/- 0.49 to 1.89 +/- 0.49 mm (p < 0.005). The diameter of internal mammary artery grafts did not change significantly, from 3.27 +/- 0.42 to 3.25 +/- 0.44 mm (-0.3 +/- 5.1%, p < 0.02 compared with saphenous vein grafts and p < 0.002 compared with coronary arteries). After isosorbide dinitrate, the diameter of both grafted coronary arteries and mammary artery grafts increased significantly (respectively to 2.46 +/- 0.62 mm and 3.44 +/- 0.43 mm), the vasodilatation being greater (p < 0.002) in coronary arteries (+17.8 +/- 9.8% in proportion to basal diameter, p < 0.001) than in mammary grafts (+5.5 +/- 3.3%, p < 0.001). The diameter of saphenous vein grafts returned to control values (3.28 +/- 0.70 mm, NS compared with basal); the changes in luminal diameter after nitrates (+0.7 +/- 3.1%, NS) were significantly smaller than for mammary artery grafts (p < 0.01) and for grafted coronary arteries (p < 0.001). CONCLUSIONS. Unlike internal mammary arte