Gastrulation is characterized by dramatic cell migration which is thought to require the interaction of cell adhesion molecules with extracellular molecules. We have tested two novel peptides, a fibronectin peptide and a fibronectin receptor peptide, for their effects on gastrulation of the leopard frog Rana pipiens. The fibronectin peptide DRVPHSRNSIT corresponds to residues 1373-1383 of the cell-binding domain of fibronectin; the receptor peptide DLYYLMDL corresponds to residues 124-131 of beta 1 subunit of a variety of integrins including alpha 5 beta 1. Either of these peptides significantly inhibited gastrulation after being microinjected into mid-blastulae. These results indicate that these sequences may correspond to the ligand/receptor interaction sites of fibronectin and its receptor(s).