Activin, Nodal, and Vg1, members of the transforming growth factor beta (TGF beta) superfamily, transduce signal through type I receptors ALK4 or ALK7 and play important roles in mesoderm induction and patterning during vertebrate embryogenesis. However, the timing and magnitude of the ALK4/ALK7-mediated maternal TGF beta signals are not clear. SB-431542 is identified as an inhibitor of the ALK4/ALK5/ALK7-mediated TGF beta signals and its specificity in vertebrate embryos has not been reported. We demonstrate that SB-431542 is able to specifically and reproducibly block the Smad.2/3-mediated TGF beta signals in zebrafish embryo. Embryos exposed to SB-431542 exhibit various defects phenocopying Nodal-deficient mutants. SB-431542 treatments starting at different cell cycles before the midblastula transition lead to different degrees of developmental defects in mesoderm induction and patterning, suggesting that maternal TGF beta signals are activated right after fertilization and required for mesoderm formation and patterning. (c) 2006 Elsevier Inc. All rights reserved.