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1. Sympathomimetic amines have been considered to be related to vasospasm. Previous studies showed that the human internal mammary artery (IMA) was capable of weak beta‐adrenoceptor mediated relaxation and that alpha‐adrenoceptor agonists may induce contraction in the human IMA. 2. We investigated the effects of glyceryl trinitrate (GTN), a vasodilator agent often used perioperatively, on alpha‐adrenoceptor mediated contraction in the human IMA. 3. Discarded human IMA segments were taken from 37 patients who underwent IMA‐coronary artery bypass graft operations and equilibrated in an organ bath. 4. A specially designed technique was used to normalize the vessel segments under the pressure similar to the in vivo situation. Noradrenaline (NA), phenylephrine (PE), and methoxamine (MO) were used to contract the vessel segments. 5. GTN fully relaxed PE or MO (submaximal concentration) induced precontraction. Therapeutic plasma concentration of GTN relaxed 40‐90% of the PE induced contraction (2.82 g, EC50 = 7.92 +/‐ 0.06 ‐log M) and 20‐90% of the MO induced contraction (1.8 g, EC50 = 7.63 +/‐ 0.16 ‐log M). Pretreatment by the therapeutic plasma concentration of GTN inhibited the contraction induced by NA, PE in a different range. It reduced the NA induced contraction (6.9 g) by 14.8‐ 38% (P greater than 0.05) and the PE induced contraction (4.3 g) by 7.9‐ 39.3% (P greater than 0.05). The alpha 1‐adrenoceptor antagonist prazosin, at the therapeutic plasma concentration, nearly totally abolished the NA or PE induced contraction (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)