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Internal mammary arteries (IMA) and saphenous veins (SV) are vessels currently used in human coronary artery bypass surgery. In addition to late complications, the vessels may develop spasm perioperatively. We studied isolated IMA and SV from patients undergoing coronary artery bypass graft to reproduce in vitro the phenomenon of vasospasm. Vascular rings were constricted with phenylephrine in a classic organ bath. The effects of two vasodilator agents, milrinone and sodium nitroprusside (SNP), on phenylephrine precontracted vessels and as a pretreatment to reverse or prevent the contraction, respectively, were studied. When added to a precontracted vessel, milrinone had the same vasorelaxant effect as SNP in artery rings (EC50: 7.4 × 10-7 ± 0.8 × 10-7 vs. 5.9 × 10-7 ± 0.8 × 10-7M, milrinone vs. SNP). In veins, milrinone was less effective in relaxing the rings than SNP (EC50: 15 × 10-7 ± 3 × 10-7 vs. 1.5 × 10-7 ± 0.1 × 10-7M, milrinone vs. SNP, p < 0.05). If milrinone or SNP was added as a pretreatment, using the EC50 values, the inhibitory effect of milrinone on phenylephrine-induced contractions was greater in arteries than in veins (71 ± 4 vs. 36 ± 11% inhibition of maximum contraction to phenylephrine, artery vs. vein, p < 0.05). In arteries, milrinone caused a greater inhibitory effect than SNP (71 ± 4 vs. 52 ± 9% inhibition, milrinone vs. SNP, p < 0.05), but similar inhibition in veins (36 ± 11 vs. 42 ± 16%, milrinone vs. SNP). These results suggest that milrinone could be useful in preventing IMA spasm and, to a lesser extent, SV spasm, but not in reversing established spasm of an artery.