The microcirculation of tumors is severely disturbed. Tumors are usually supplied by fragile capillaries and do not possess the natural hierarchy of blood vessels. The detection of specific markers for arterial and venous endothelial cells (ECs) now enables us to study the vascular tree in tumors. We have injected rat C6 glioma and human A375 melanoma cells into 3.5- to 4-day-old avian embryos. After 10-12 days of reincubation the tumor cells formed solid tumors vascularized by host ECs. In contrast to the melanomas, the gliomas induced an almost normal vascular tree with arterial and venous vessels. The arterial vessels express the arterial EC marker ephrin-B2, and possess a media of smooth muscle alpha-actin (alphaSMA)-positive cells. Venular vessels in the gliomas are ephrin-B2-negative/alphaSMA-positive. Although the gliomas may represent a rare case of vascular tree induction in tumors, the results underline the heterogeneity of tumor-induced angiogenesis. This has an impact on tumor blood flow and thereby also on the efficacy of chemotherapy and radiotherapy.