The Spemann organizer has long been recognized as a major source of patterning signals during the gastrula stage of amphibian embryogenesis. More recent evidence has suggested that the ventral side of the embryo also plays an important role in dorsal-ventral patterning during gastrulation through the action of signaling factors such as BMP-4. Bmp-4 is closely related to the Drosophila decapentaplegic (dpp) gene, and like Bmp-4, dpp is excluded from the neurogenic region. Recently we showed that Bmp-4 functions in an analogous role to that of dpp in Drosophila, suggesting that the mechanism of dorsal-ventral patterning in Xenopus and Drosophila embryos may be conserved. To further test this hypothesis, RNA of the Drosophila short gastrulation (sog) gene was injected into Xenopus embryos, since sog has been shown genetically to be an antagonist of dpp function. Overexpression of sog RNA in Xenopus dorsalizes the embryo by expanding neurogenic and dorsal paraxial tissue. When ectopically expressed on the ventral side of the embryo, sog induces a partial secondary axis. In addition, sog partially rescues embryos ventralized by ultraviolet irradiation. Since sog induces many similar changes in gene expression to that caused by truncated BMP receptors, we suggest that sog functions in part by opposing BMP-4 signaling. The recent identification of a possible Xenopus sog homolog, chordin, in conjunction with these results supports the hypothesis that dorsal-ventral patterning mechanisms are conserved between these two species.