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The dorsoventral axis is established early in Xenopus development and may involve signaling by Wnts, a family of Wntl-protooncogene-related proteins. The protein kinase shaggy functions in the wingless/Wnt signaling pathway, which operates during Drosophila development. To assess the role of a closely related kinase, glycogen synthase kinase 3,B , in vertebrate embryogenesis, we cloned a cDNA encoding a Xenopus homolog of To assess a possible role for GSK-3,B in dorsoventral axis formation, we cloned a Xenopus homolog of GSK-3,B/shaggy (XGSK-313)t and examined its expression pattern in early embryos. We also constructed several mutated forms of rat GSK-3,3 and studied effects of their overexpression in Xenopus embryos. Since shaggy is a presumed negative regulator of the Wg pathway in Drosophila, we speculated that a dominant negative mutant of GSK-3P may mimic the effect of ectopic Wnt expression on embryonic axis formation. Consistent with these expectations, microinjection of an enzymatically inactive form of rat GSK-3f3 carrying a point mutation in the ATPbinding site triggered an ectopic body axis formation in early embryos. This effect could be suppressed by the wild-type XGSK-3,3. Our data strongly indicate that the endogenous