While there is convincing evidence implicating Drosophila int-1 in pattern regulation, the normal role of int-1 in vertebrate development is unclear. We have injected Xenopus eggs with mouse int-1 RNA and monitored subsequent development. Injected RNA is translated and the protein widely distributed. Embryos develop into apparently normal gastrulae, but almost all surviving neurulae have a bifurcated anterior and expanded posterior neural plate. Bifurcation of the neural plate was abolished by substitution of a single, conserved cysteine residue and was dependent on the presence of a signal peptide sequence in the int-1 protein. Histological examination indicates that underlying axial mesodermal structures were duplicated. This result suggests that ectopic int-1 expression leads to dual axis formation and points to a role for int-1 in patterning processes in vertebrate development.