The mutant mouse strains splotch-delayed (Pax3Sp-d) and curly tail (ct) develop neural tube defects (NTDs) in the lumbosacral region of the neuraxis. Some research has focused on cell proliferation around the time of posterior neuropore closure in these mutants; however, there are little data on the effects of NTDs on cell birth at later stages of development. To investigate the role neural tube closure might play in cytogenesis of the spinal cord, the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) was injected into pregnant splotch-delayed and curly tail mice at various stages of gestation. The mean number of labelled cells in the dorsal and ventral halves of spina bifida and control embryos was then calculated per section and per mm2. Mutagenically separated PCR (MS-PCR), was used to ascertain the genotype of splotch-delayed embryos. Our data indicate that the peak proliferation dates, for both the dorsal and ventral regions of the cord, are similar in spina bifida and control embryos. However, the quantity of proliferation is significantly different between affected and unaffected embryos. In general, there are markedly fewer cells born in spina bifida embryos in early neural tube development, followed by a short period of equal proliferation, and culminating in a significant increase in cell proliferation later in gestation. This increase in proliferation results in a greater number of cells being born in spina bifida embryos compared to controls. Several possible explanations for this phenomenon are considered, including the hypothesis that the roof plate, or other factors induced by neural tube closure, might have an anti-mitotic activity.