The internal mammary artery is currently regarded as the optimal canal in coronary bypass surgery. Even if the motives for the excellent behaviour of this artery are still not fully clarified, the morphological and morphofunctional study of its wall can contribute to explain the clinical results obtained using this canal. In order to complete existing studies on this topic and to increase our knowledge of the structure of this artery, segments of internal mammary artery taken from patients undergoing coronary bypass surgery were analysed. Histological, immunohistochemical, histochemical and morphometrical tests were performed in these samples. The results of the tests underline the considerable complexity of the internal mammary artery whose walls appear to be rich in elastic fibres, divided into numerous lamellar structures concentric to the vasal lumen and without vasa vasorum in the tunica media given that this is adjacent to the tunica adventitia. Moreover, the marked presence of the NO-synthase enzyme, responsible for nitrogen monoxide synthesis, was observed in the endothelial layer and tunica media, as suggested by histochemical analysis. These data argue in favour of a structure able to resist the considerable hemodynamic stress to which the artery is subjected after bypass, a structure which is functionally well adapted to receive the majority of nutrition through its own lumen, and, lastly, a structure capable of self-regulation in response to the requirements made from time to time. These findings may further our understanding of the morphofunctional aspects of the internal mammary artery and may contribute to explaining the latter's relative immunity to atherosclerosis and, thus, its success in coronary bypass.