Activin, a peptide growth factor related to tumour growth factor-beta, has been implicated in early inductive interactions in vertebrates and can induce Xenopus blastula ectodermal explants to develop a rudimentary axial pattern with anteroposterior and dorsoventral polarity. Here we demonstrate that prospective dorsal and ventral regions of the ectoderm respond differently to the same concentration of activin. Thus, activin does not seem to endow ectodermal cells with polarity but rather reveals a pre-existent pattern. Our results suggest that patterning of mesoderm is determined not only by a localized inducer, but also by the differential competence of cells in the responding tissue.