Neuralization (archencephalic brain formation) takes place after dissociation and delayed reaggregation of animal caps of early gastrula without inducer (Grunz, H. and L. Tacke: Cell Differ. Dev. 28, 211-218 (1989)). This autoneuralization can be prevented by the cell supernatant from dissociated ectoderm of Xenopus laevis, which contains extracellular matrix components. After phenol extraction of the supernatant, the aqueous phase does no longer show inhibitory activity. It can be concluded from these results that glycoconjugates responsible for the prevention of neuralization represent glycoproteins or proteoglycans which are loosely attached to integral plasma membrane components. Single early gastrula ectoderm cells mixed with non-competent late gastrula ectoderm or endoderm, which primarily form common aggregates, do not differentiate into neural derivatives. In these reaggregates the ectoderm cells remain separated from each other by heterologous cells (non-competent ectoderm or endodermal cells) during the period of competence. These data indicate that the quick recovery of extracellular matrix components together with the restoration of the former organization of the plasma membrane is responsible for the prevention of neuralization.