ABSTRACT The neural cell adhesion molecule (NCAM) is one of the most abundant cell adhesion molecules expressed in vertebrates and it is thought to play important roles as a regulator of morphogenetic processes, but little is known of its expression pattern in mammalian embryos. In this study, we have examined the developmental profile of NCAM gene expression in mouse embryos from gestational day 7.5 to 12.5, focusing on the developing neural tube. NCAM transcripts were first detected around day 8.5 in the somites and the forming neural tube. At this stage, NCAM transcripts were expressed in the neuroepithelium throughout the width of the neural groove and tube up to a rostral boundary within the hindbrain, whereas NCAM mRNA levels were very low or undetectable in the neuroepithelium of the head region. The positional restriction of NCAM expression was con-firmed by immunohistochemistry at the protein, and by polymerase chain reaction analysis at the RNA level. Expression in the neuroepithelium was transient as the level of NCAM transcripts declined in the germinal layer beyond day 8.5. By day 9.5, strong NCAM expression had appeared on the earliest postmitotic neurones along the entire neuraxis, and this pattern of expression in all regions with differentiating neurones was maintained until day 12.5. We conclude that NCAM expression in the neural tube occurs in two spatiotemporal distinct waves : a first wave in the proliferating neuroepithelium showing positional dependence along the rostrocaudal axis, and a second wave on essentially all neurones that have become postmitotic.