Regional patterns of deposition of laminin (LN), fibronectin (FN), type IV collagen (IV), and heparan sulfate proteoglycan (HSPG) were examined during the formation of the caudal neural tube in embryos homozygous for the delayed Splotch gene and in their normal littermates. Delayed Splotch embryos had neural tube closure defects which extended from the posterior neuropore into the region formed by secondary neurulation. During posterior neuropore closure these components were normally restricted to forming basal laminae, with FN and HSPG additionally deposited in the mesenchyme. Unlike control embryos in which medial regions of the neuroepithelial basal lamina contained greatest amounts of all four, the dorsolateral zone contained less LN and IV and more FN and HSPG, in affected embryos these components were less densely deposited medially, reflecting perhaps the poor structural organization of the notochord. The neuroepithelial basal lamina was often disorganized and wavy compared to the linear pattern typical of controls. By the 12th day, the posterior neuropore of controls had closed and secondary neurulation was underway; however in delayed Splotch embryos, the neural folds remained widely splayed and epithelium newly formed via secondary neurulation extended that abnormally open configuration to the tip of the tailbud. In controls, with mesenchymal cell aggregation FN and HSPG were displaced from between cells to the forming basal lamina. As a central lumen formed within the aggregate LN and IV were added to the basal lamina, and the newly formed epithelium merged with the anterior neural tube. In delayed Splotch embryos, FN and HSPG were incompletely removed from aggregating cell surfaces, the normal morphogenetic cell shaping changes failed to occur and in many embryos a central lumen did not form; the overgrown, aggregated cells merging with the abnormally splayed anterior neural folds. In addition, the critical enrichment of FN and HSPG present between newly formed and consolidated neuroepithelium was displaced in delayed Splotch embryos.