Activin/nodal-like TGF-beta superfamily ligands signal through the type I receptors Alk4, Alk5, and Alk7, and are responsible for mediating a number of essential processes in development. SB-431542, a chemical inhibitor of activin/nodal signaling, acts by specifically interfering with type I receptors. Here, we use inhibitor-resistant mutant receptors to examine the efficacy and specificity of SB-431542 in Xenopus and zebrafish embryos. Treatment with SB-431542 eliminates Smad2 phosphorylation in vivo and generates a phenotype very similar to those observed in genetic mutants in the nodal signaling pathway. Inhibitor-resistant Alk4 efficiently rescues Smad2 signaling, developmental phenotype, and marker gene expression after inhibitor treatment. This system was used to examine type I receptor specificity for several activin/nodal ligands. We find that Alk4 can efficiently rescue signaling by a wide range of ligands, while Alk7 can only weakly rescue signaling by the same ligands. In whole embryos, nodal signaling during gastrulation can be rescued with Alk4, but not Alk7, while Alk5 can only mediate signaling by ligands expressed later in development. The combination of the ALK inhibitor SB-431542 with inhibitor-resistant ALKs provides a powerful set of tools for examining nodal/activin signaling during embryogenesis.