Basement membranes (BM) are specialized structures of the extracellular matrix known to be involved in various early developmental processes. Despite numerous investigations on the localization of BM components, it remains unknown which molecules are expressed in early developmental stages and by which germ layers these proteins are produced. Therefore, we tested for all known laminin chains, nidogens, collagen type IV, and perlecan by means of light microscopic immunostaining and performed in situ reverse transcriptase-polymerase chain reaction to detect the mRNAs specific for laminin α1, laminin β1, the α1 chain of collagen type IV, nidogen-2, and perlecan in the early mouse embryo, day 7, in vivo. Only the laminin chains α1, β1, and γ1 were detected immunohistochemically throughout the entire endodermal and ectodermal BM zones of the embryo proper. The mRNA of laminin α1, laminin β1, collagen type IV, nidogen-2 and perlecan were expressed in the ectoderm-derived mesoderm, in the endoderm as well as in the ectoderm. In contrast, Reichert's membrane was positive for all laminin chains except for the α4, α5, β3, and γ3 chains. Moreover, maternal epithelial as well as mesenchymal cells expressed laminins, nidogen-1 and nidogen-2, collagen type IV, and perlecan. In conclusion, laminin-1 might be the only laminin isoform in the early mouse embryo that, together with the other main BM components, nidogens, collagen type IV, and perlecan, is synthesized by all three germ layers. Developmental Dynamics 233:1140–1148, 2005. © 2005 Wiley-Liss, Inc.