Protein A-gold immunocytochemistry was applied in combination with morphometrical approaches to reveal the alpha 1(IV), alpha 2(IV), and alpha 3(IV) chains of type IV collagen as well as entactin on renal basement membranes, particularly on the glomerular one, during maturation. The results have indicated that a heterogeneity between renal basement membranes appears during the maturation process. In the glomerulus at the capillary loop stage, both the epithelial and endothelial cell basement membranes were labeled for the alpha 1(IV) and alpha 2(IV) chains of type IV collagen and entactin. After fusion, both proteins were present on the entire thickness of the typical glomerular basement membrane. At later stages, the labeling for alpha 1(IV) and alpha 2(IV) chains of type IV collagen decreased and drifted towards the endothelial side, whereas the labeling for the alpha 3(IV) chain increased and remained centrally located. Entactin remained on the entire thickness of the basement membrane during maturation and in adult stage. The distribution of endogenous serum albumin in the glomerular wall was studied during maturation, as a reference for the functional properties of the glomerular basement membrane. This distribution, dispersed through the entire thickness of the basement membrane at early stages, shifted towards the endothelial side of the lamina densa with maturation, demonstrating a progressive acquisition of the permselectivity. These results demonstrate that modifications in the content and organization of the different constituents of basement membranes occur with maturation and are required for the establishment of the filtration properties of the glomerular basement membrane.