The myocardial bridge (MB) is a muscle band found sporadically above the coronary artery (CA) in humans and certain animals such as the dog, cat and sheep. The purpose of our study was to compare the structure of the MB muscle with that of tissue from the subepicardial myocardium. The histological studies included toluidine blue staining of 1-micron-thick sections and Gomori's trichrome staining of canine cardiac samples. The MB muscle of the dog heart is characterized by a distinctive spatial arrangement, with individual fibers separated by substantial elements of intercellular connective tissue in cross-section. Longitudinally, the long, slender fibers are aligned continuously with intermediation of intercalated disks lying perpendicular to the long axis of the fibers. In other regions of the left ventricular subepicardial myocardium, each myocyte is tightly packed in transverse view. There is great variation in the thickness (0.11-2.24 mm, average 0.45 mm) of MBs and the distance (24-236 microns, average 103 microns) between MBs and the left anterior descending coronary artery (LAD) among the 13 affected dogs examined, with no apparent relationship between the occurrence of MBs and either age or sex. These results on MB alignment suggest that the MB muscle generates force along the long axis of the fiber orientation as skeletal muscle does, and with minimal constriction of the CA; if so, the function of MB myocytes may differ from that of common cardiac myocytes, as does the structure. Then, the long-supposed downward compressive force of MBs on the LAD would be minimal in most cases; however, when the MB produces a systolic narrowing of the LAD known as the milking effect, the degree of lateral compression and its influence should depend not only on the substantial size of the MB muscle but also on the distance between the MB muscle and LAD. The environment surrounding the LAD may be a crucial factor in determining whether the MB influences the induction of heart disorders or not.