Antibodies raised against the major 27 kDa protein electrophoretically eluted from isolated gap junctions and affinity purified against the antigen have been used to explore the role of communication through gap junctions in the early amphibian and mouse embryos. In both species, injection of the antibodies into one cell completely blocks both dye transfer and electrical coupling between cells connected by gap junctions. In the amphibian embryo the generation of a communication-incompetent clone of cells leads to patterning defects in the region derived from the antibody-injected cell. In the mouse embryo, blocking cell-to-cell communication leads to decompaction of the communication-incompetent cells. The possible significance of these findings in relation to development in general and to the organization of the first transporting epithelia to appear during development is discussed.