Studies in Xenopus have shown that growth factors of the TGF beta and Wnt oncogene families can mimic aspects of dorsal axis formation. Here we directly compare the inductive properties of two Wnt proteins by injecting synthetic mRNA into developing embryos. The results show that Wnt-1 and Xwnt-8 can induce a new and complete dorsal axis and can rescue the development of axis-deficient, UV-irradiated embryos. In contrast, activin mRNA injection induces only a partial dorsal axis that lacks anterior structures. These studies demonstrate that the mechanism of Wnt-induced axis duplication results from the creation of an independent Spemann organizer. The relationship between the properties of the endogenous dorsal inducer and the effects of Wnts and activins is discussed.