The bursa of Fabricius is a lymphoid organ of the chick which plays an important role in the development of the immune system. The role of angiogenic factors in the development of the vascular system of this organ has been poorly investigated. Vascular endothelial growth factor (VEGF) is a major regulator of endothelial cell proliferation, angiogenesis and vascular permeability, and its activities are mediated by two receptors, VEGFR-1 and VEGFR-2. In this study we have investigated by immunohistochemistry the VEGF and VEGFR-2 immunoreactivity in developing bursa of Fabricius. Starting from day 10 of incubation, the endodermal epithelium reacts with VEGF and gives rise to the lymphoid follicles, while the vascular endothelium reacts with VEGFR-2. These data support the view that VEGF acts as a paracrine stimulator of angiogenesis in the avian embryo and confirm the requirement of the endodermal layer for the normal formation of blood vessels by mesodermal cells.