Splenic vascular casts of the common tree shrew, Tupaia glis, were constructed with Batson's No. 17 plastic mixture and studied with the scanning electron microscope (SEM). Fifteen adult animals of both sexes, weighing between 120 and 180 g were used. Under ether anaesthesia, each animal was injected with 0.05 ml heparin intracardially; the right atrium was cut open and then 250 ml of 0.9% NaCl, followed by 50 ml of 10% neutral formalin, (in four animals) was injected through the left ventricle. Plastic mixture was injected through the same opening. After complete polymerization of the plastic, the spleen and surrounding tissues were removed and macerated in 40% KOH. The air-dried casts were then coated with carbon and gold before viewing and photographing under SEM at 15 kV. It was found that the splenic arteries penetrated deep into the organ before they divided into trabecular arteries and divided again into central arterioles. Each central arteriole sent out 15 to 30 radiating arterioles, called penicillar arterioles, and further divided into smaller vessels entering the marginal zone and red pulp. In this area each arteriole continued directly into either marginal or red pulp sinusoids. The sinusoids emptied into pulp venules which joined to form trabecular veins. Most of the trabecular veins travelled to the cortical area underneath the splenic capsule before approaching the hilum, where they finally drained into splenic and short gastric veins. It is likely that the spleen of the common tree shrew has a closed circulation.