Kidney glomerulus mesangial cells fail to develop in mice carrying targeted null mutations in the platelet-derived growth factor (PDGF)-B or PDGF-Rβ genes. We have examined the pattern of expression of these genes and smooth muscle markers during kidney development, to address the possible mechanisms underlying the mutant phenotypes. In wild-type embryos, PDGF-B was expressed in vascular endothelial cells, particularly in capillary endothelial cells in the developing glomeruli, whereas PDGF-Rβ was found in perivascular mesenchymal cells in the developing renal cortex. In the course of glomerular development, small groups of PDGF-Rβ and desmin-expressing cells collected in the ‘S’-shaped and early cup-shaped vesicles, and at later stages such cells were found in the glomerular mesangium. In PDGF-B or -Rβ null embryos, some PDGF-Rβ/desmin or desmin-positive cells, respectively, were seen in early cup-shaped vesicles, but fewer than in the wild type, and further development of the mesangium failed. In mouse chimeras composed of PDGF-Rβ +/+ and −/− cells, the Rβ−/− cells failed to populate the glomerular mesangium. Our results show that while the mesangial cell lineage is specified independently of PDGF-B/Rβ, these molecules provide critical permissive signals in mesangial cell development. We propose a model in which mesangial cells originate from PDGF-Rβ-positive progenitors surrounding the developing glomerular afferent and efferent arterioles, and are co-recruited in response to PDGF-B during angiogenic formation of the glomerular capillary tuft.