Vascularization and endothelial phenotype were investigated in embryonic tissues grafted onto chorioallantoic membrane (CAM) by means of immunocytochemistry and electron microscopy. Single grafts of adrenal gland or cerebellum and double grafts of adrenal gland plus cerebellum were performed, using tissues from chick or quail embryos as donors and CAMs of chick embryos as recipients. Vessels of quail origin were discriminated from those of chick origin by the anti-MB1 monoclonal antibody, specific for antigenic determinants of the quail endothelial cells. The cerebellum endothelia were distinguished from the adrenal and CAM endothelia by a polyclonal antibody against the isoform 1 of the glucose transporter (GLUT1), which is a marker of barrier-provided brain vessels. The observations, carried out, 6 days after implantation, revealed the new-growth of microvessels from the CAM into the grafted tissues, and vice versa, in both single and double transplants. In addition, in the double grafts, adrenal-derived vessels were seen to grow into the cerebellum and cerebellum-native vessels into the adrenal tissue. The combined immunocytochemical and electronmicroscopical study demonstrated that the adrenal, fenestrated sinusoids and the cerebellar, barrier-provided capillaries maintain their original phenotype when they grow within the non-native tissues. The conventional theory on the endothelial responsiveness to environmental signals has been discussed and some concluding remarks have been made.