Cardiac musculature of the cranial vena cava in the common tree shrew (Tupaia glis) was examined by light and transmission electron microscopy. The common tree shrew has well developed cardiac myocyte layers in the tunica media of the cranial vena cava, extending from the right atrium to the root of the subclavian vein. Because the common tree shrew belongs to a primitive group of mammals, the occurrence of cardiac musculature in the cranial vena cava may be a common feature in lower mammals. The development of this musculature indicates that active contraction of the cranial vena cava wall occurs in this species. Electron micrographs showed the typical ultrastructure of myocytes and nerve endings. These observations suggest that this musculature may serve as a regulatory pump for the return of venous blood to the right atrium and as a blood reservoir system under conditions of rapid heart rate. Additionally, the presence of atrial natriuretic polypeptide (ANP) was also demonstrated in the myocytes of the vena cava immunohistochemically. These findings show that the cardiac endocrine organ for ANP develops even in the principal veins including the cranial vena cava.