The process by which the vertebrate central nervous system acquires its regional properties remains a central problem in developmental biology. It is generally argued that at early gastrula stages the dorsal mesoderm possesses precise anterior-posterior positional information, which is subsequently imparted to the overlying ectoderm. However, using regionally specific gene probes to monitor regional responses in Xenopus embryos, we find that anterior-posterior properties are not fixed until early neurula stages. During gastrulation the regional inducing capacities of the dorsal mesoderm as well as the regional responses of the presumptive neural ectoderm are activated along the entire anterior-posterior axis when these properties are assayed in recombinant and explant experiments, respectively. Restriction of regional inducing capacity in the mesoderm and responsiveness in the neural ectoderm occur only at neural plate stages.