FGF signaling has been implicated in germ layer formation and axial determination. An antibody specific for the activated form of mitogen-activated protein kinase (MAPK) was used to monitor FGF signaling in vivo during early Xenopus development. Activation of MAPK in young embryos is abolished by injection of a dominant negative FGF receptor (XFD) RNA, suggesting that MAPK is activated primarily by FGF in this context. A transition from cytoplasmic to nuclear localization of activated MAPK occurs in morula/blastula stage embryo animal and marginal zones coinciding with the proposed onset of mesodermal competence. Activated MAPK delineates the region of the dorsal marginal zone before blastopore formation and persists in this region during gastrulation, indicating an early role for FGF signaling in dorsal mesoderm. Activated MAPK was also found in posterior neural tissue from late gastrulation onward. Inhibition of FGF signaling does not block posterior neural gene expression (HoxB9) or activation of MAPK; however, inhibition of FGF signaling does cause a statistically significant decrease in the level of activated MAPK. These results point toward the involvement of other receptor tyrosine kinase signaling pathways in posterior neural patterning.