The coronary sinus is an anatomic landmark and conduit for many diagnostic and therapeutic procedures. We studied 40 human and 40 canine coronary sinuses of the heart in order to ascertain various functional anatomic features of the coronary sinus and its tributaries. We performed a comparative analysis of human and canine cardiac veins. We also studied the formation, tributaries, length, diameter and the ostium of the coronary sinus. The tributaries of the coronary sinus were highly variable in human and dog. The main and constantly present tributaries of both the human and canine coronary sinus were the great and the middle cardiac veins. We also found a rare case of a small cardiac vein connecting two cardiac venous systems--the coronary sinus and its tributaries and the anterior cardiac veins. In two cases we found a common trunk of the small cardiac vein and middle cardiac vein which opened into the coronary sinus. An intramyocardial course of the anterior interventricular vein was found in 2 cases and in 1 case the vein curved twice around the anterior interventricular branch of the left coronary artery. The Thebesian and Vieussens valves and ostial valves of other cardiac veins were also found.