Thebesian vasculature provides for communication between the coronary system and the chambers of the heart. Anatomic, embryologic, physiologic, and therapeutic investigations have involved this component of cardiac anatomy from the early 18th century to the present time. The scanning electron microscope (SEM) now affords an innovative approach to the study of the ostia of these veins as they open into the chambers of the heart. The surface of the intact endocardium is continuous, whether it is treated with boric acid or not, as long as it remains intact. Enzymatic microdissection of tissues with trypsin, hyaluronidase and pronase, followed by similar treatment with boric acid, reveals continuity of successive component layers of the endocardium extending into Thebesian substructure. Thebesian tributaries are easily visualized from the ostia but the deeper capillary network of the Thebesian system is not demonstrable by this approach. Valvular structures such as might prevent retroflow during the cardiac cycle are not present. Our observations with SEM support anatomic relationships indicated by previously published work.