To characterize the configuration of “tunnel capillaries” in myocytes, an ultrastructural three-dimensional (3-D) study was performed on pressure-overloaded rat hearts induced by 16-week constriction of the ascending aorta. The hearts showed high incidence (6.1 ± 3.6%) of tunnel capillary formation. Eighty-two myocytes (26.4%) had tunnel capillaries in a cubic block of 120 × 656 × 446 μm3, which provided 1000 ultrathin serial sections. The cross-sectional area (925 ± 226 μm2) of myocytes with tunnel capillaries was significantly larger than that of myocytes without tunnel capillaries (702 ± 196 μm2) (P< 0.0001). There were three types of tunnel capillaries. Type I (13%) started from one intercellular capillary, ran across the myocyte, and then merged with other intercellular capillaries. Type II tunnel capillaries (87%) entered the myocyte from one intercellular capillary and ended within the myocyte. Type III tunnel capillaries were characterized by various combination of types I and II, forming a tunnel capillary network. Tunnel capillaries usually entered a myocyte at the place where the myocyte split or indented. Although some tunnel capillaries might be newly formed by angiogenesis, our 3-D study suggests that some of them are a deformation consequence of the myocardium remodeling in response to pressure overloading.