An immunohistochemical survey was carried out on frozen sections of the early embryonic chick brain between 1 and 6 days of incubation, with antisera to the three neurofilament proteins (NF-L, NF-M, NF-H). Large numbers of replicating neuroepithelial cells were found to express one of these proteins, NF-M, generations before the existence of any postmitotic neuroblasts (Days 1-2 1/2 of incubation). NF-L and NF-H could not be detected. Not all primordial brain regions contained NF-M-positive cells, but in those that did, every cell was positive. These regions included the dorsal forebrain, optic vesicles, and dorsal hindbrain, but not the dorsal midbrain. All cells in all regions of the cephalic neural tube contained vimentin, whether or not they also contained NF-M. This NF-M expression was transient in the sense that later generations of these NF-M-positive neuroepithelial cells became NF-M negative, before finally giving rise to some descendents that ultimately express all three NF proteins. This transient NF-M expression was found in certain other cells of early embryos, including cardiac myoblasts. The identity of the component in these early neural and nonneural tissues, that bound the antibody, was demonstrated to be identical to adult brain NF-M by one- and two-dimensional immunoblots. These findings demonstrate an unusual kind of biochemical heterogeneity among neuroepithelial cells, and they are relevant to considerations regarding lineage analysis and lineage "markers" in the vertebrate central nervous system.