The ultrastructure of the terminal vascular bed of human coronary arteries was studied in the myocardial tissue obtained at surgery from different locations in the heart in five patients. The following vessels were identified: (1) Arterioles; slender and prolonged endothelial cells, flat nuclei and two to three layers of smooth muscle cells. (2) Precapillary sphincters: short endothelial cells, large nuclei bulging into the lumen, close myoendothelial junctions and a single layer of circular smooth muscle. (3) Capillaries: composed of one or more slender endothelial cells. (4) Venules: flat endothelial cells and nuclei, no muscular layer, rich collagen tissue. The function of these structures is believed to be as follows: the arterioles are the smallest blood-distributing arteries in the heart. The precapillary sphincters control blood flow to the capillaries; pressor substances present in the blood are picked up by endothelial cells, pass rapidly through the myoendothelial junctions and cause contractions of the smooth circular muscle layer; the bulging nuclei of endothelial cells then passively obstruct the lumen almost completely. The main exchange of gases and nourishing substances takes place in the capillaries. We postulate that in some pathologic conditions, abnormal constriction of the sphincters may cause diminished flow and be the basis for some well defined or unclear ischemic events.