The murine Brachyury (T) gene is required in posterior mesoderm formation and axial development. Mutant embryos lacking T gene function are deficient in notochord differentiation and posterior mesoderm formation, but make anterior mesoderm. Posterior axial development requires increasing T activity along the rostrocaudal axis. The T gene is transiently transcribed in nascent and migrating mesoderm and continuously in the notochord. The maintenance of T expression in the notochord depends, directly or indirectly, on wild-type T activity. In Xenopus it has been shown that the onset of T expression occurs in response to mesoderm-inducing growth factors. The T protein is binding to DNA and is probably involved in the control of gene expression. Here we show that the T protein is located in the nucleus. We have analyzed the expression pattern of T protein in wild-type and mutant embryos from early primitive streak formation to the end of the tail bud stage. Throughout all stages of mesoderm formation T protein is transiently present in nascent and migrating mesoderm. In the notochord T protein persists to the end of the tail bud stage. It is also transiently detectable in the forming gut endoderm and in prospective neuroectoderm of later embryos. This shows that T expression is not strictly correlated with a commitment of cells to mesoderm. The analysis of the tail development of TWis/+ mutant embryos demonstrated that the formation of the neural tube, gut, and somites from the tail bud proceeds in the absence of a notochord. The maintenance and differentiation of these structures, however, seems to depend on signals from the notochord.