A dorsalizing signal acts during gastrulation to change the specification of lateral mesodermal tissues from ventral (blood, mesenchyme) to more dorsal fates (muscle, heart, pronephros). This signal, from Spemann's organizer, cannot be mimicked by the mesoderm inducers activin and fibroblast growth factor. The gene noggin is expressed in the organizer, and could be the dorsalizing signal. Here we show that soluble noggin protein added to ventral marginal zones during gastrulation induces muscle, but that activin does not. Dorsal pattern can be partially rescued in ventralized embryos by injection of a plasmid that expresses noggin during gastrulation. The results suggest that the noggin product may be the dorsalizing signal from the organizer.