The renal glomerular basement membrane (GMB) separates two distinctly different cell layers: the vascular endothelium, and visceral epithelial podocytes. When initial vascularization of the forming glomerulus takes place during nephrogenesis, the early GBM forms by fusion of a dual basement membrane between endothelial cells and podocytes. As glomerular capillary loops blossom, newly synthesized basement membrane segments derived from podocytes are then inserted or spliced into the fused GBM. The molecular processes accounting for either basement membrane fusion or splicing are unresolved. Using monoclonal anti-mouse laminin antibodies (mAbs) against the end of the laminin long arm (5D3), we have shown in adult mice that peripheral loop GBM is only weakly immunoreactive but the mesangial matrix and tubular basement membrane (TBM) is intensely positive. In contrast, mAbs against domains in the center of the laminin cross only label TBMs and mesangial matrices of mature mice and GBMs are negative. Immunofluorescence microscopy of neonatal mouse kidneys showed, however, that anti-laminin mAbs brightly labeled developing GBMs of glomeruli undergoing initial vascularization and capillary loop formation. Post-fusion GBMs of maturing stage glomeruli became unreactive for most anti-laminin mAbs but remained positive for 5D3. Our results therefore show that some GBM laminin epitopes are transiently expressed during glomerular development. These changes in GBM immunoreactivities may reflect proteolytic processing during basement membrane fusion and splicing, or temporally controlled synthesis of different laminin isoforms.