The origin of the glomerular endothelium has remained unsettled, although it has mostly been assumed that it is derived from the metanephric mesenchyme, like, the other parts of the nephron. Since our recent observations did not support this concept, the development of the vasculature of the kidney and of the glomerular endothelium was studied using both mouse/quail and quail/chick interspecies grafts. Undifferentiated 11-day mouse kidney anlagen were transplanted onto quail chorioallantoic membrane and allowed to develop for 7-10 days. Upon examination, grafts were well-vascularized, and both the glomerular and the vessel endothelium expressed the quail nuclear marker. Similar results were obtained in quail/chick transplantation experiments. Hence, we suggest that the endothelium of the glomeruli is derived from outside vasculature rather than by conversion of cells in the nephrogenic mesenchyme.