Tissue plasminogen activator (tPA) expression during embryogenesis was determined by in situ hybridization of whole mouse embryos from E10.5 through E17.5. The strongest expression occurs in the basal midbrain and hindbrain, and continues posteriorly into the neural canal. This expression coincides with extensive cell migration and proliferation, and tissue remodelling of this region. tPA mRNA is also associated with cells that appear to be invading the cerebellar anlage. Presumptive proliferating and migrating cells in the olfactory neuroepithelium also express tPA. These results indicate that tPA is expressed by a number of different cell types in the developing nervous system and suggest a role for tPA in cell migration and tissue remodelling of the developing CNS.