The expression of the neural cell adhesion molecule (N‐CAM) and α 2–8 linked polysialic acid (PSA), which is believed to be predominantly expressed on N‐CAM, was investigated during early embryonic development of the mouse (embryonic days 7.5 to 10.0). By immunocytochemistry, in tissue sections, N‐CAM and PSA were not detectable at embryonic day 7.5 but were expressed in the prominent body regions such as somites, unsegmented mesoderm, developing heart, and neuroectoderm at embryonic day 8.0 N‐CAM and PSA immunoreactivities were always predominantly associated with the plasma membrane. No tissue could be detected which was positive for PSA but negative for N‐CAM. In Western blot analysis of whole embryos, by contrast, only the lightly sialylated and PSA‐negative 180 and 140 kD isoforms of N‐CAM were present at embryonic day 8.0 and strong expression of PSA‐bearing, heavily sialylated N‐CAM was not detectable before embryonic day 10.0. In Western blot analysis of N‐CAM immunoaffinity purified from whole embryos and digested with neuraminidase as well as in Northern blot analysis, the 120 kD isoform of N‐CAM or its corresponding mRNA were not expressed in detectable amounts during the time period investigated. © 1994 Wiley‐Liss, Inc.