To address origins of glomerular endothelial and mesangial cells in embryonic mammalian kidneys, we established interspecies grafts between rats and mice, in which fetal kidneys were implanted into the anterior eye chamber of adult hosts. After 5-7 days, hosts bearing grafts received intravenous injections with species-specific monoclonal antibodies (MAbs) to matrix components. In all cases, glomerular basement membranes and mesangial matrices labeled solely for donorderived matrix. Additionally, microvessel extracellular matrices within grafts were usually of donor origin. To examine directly the origin of glomerular endothelial and mesangial cells, we grafted embryonic gestational days 11-12 (E11-12) kidneys from normal mice into anterior eye chambers of host reverse-orientation splice acceptor 26 mice, which are transgenic animals that express β-galactosidase in every cell. When grafts were developed for β-galactosidase activity, host cells were seen in peripheral vessels, but the majority of glomerular endothelial cells were of donor, not host, origin. Where host-derived endothelial cells were found in glomeruli, donor endothelial cells were present as well. Mesangial cells were always of donor origin. When Ell mouse kidneys were labeled with the endothelial cell-specific Bandeiraea simplicifolia isolectin 64, we determined that endothelial cells are present from the inception of metanephrogenesis. Together, the evidence shows that cells of endogenous kidney origin were almost entirely responsible for development of the glomerular microvasculature in oculo. External vessels from the host, although important for graft maintenance, were not major contributors to the glomerulus. angiogenesis; basement membrane; vasculogenesis Copyright © 1996 the American Physiological Society.