De novo assembly of gap junctions begins during compaction in the eight-cell stage of mouse development, and intercellular coupling mediated by gap junctions appears to be required for maintenance of the compacted state. We have begun to explore the expression of the family of genes encoding the connexins, the proteins that form the gap junction channels. We recently reported that a protein with antigenic and size similarity with connexin32, the rat liver gap junction protein, is inherited as an oogenetic product by the mouse zygote, but its gene appears not to be transcribed prior to implantation (Barron et al., Dev Genet 10:318-323, 1989). Here we report that another member of this gene family, connexin43, is transcribed by the embryonic genome from shortly after the time of genomic activation. As revealed by Northern blotting, connexin43 mRNA is absent from ovulated oocytes, becomes detectable in the 4-cell stage, and accumulates steadily thereafter to reach a maximum in blastocysts. In contrast, no transcripts of connexin26 could be detected in any preimplantation stage. A protein with antigenic and size similarity with connexin43 from rat heart was found by Western blotting to accumulate from the four-cell stage onward. Immunofluorescence analysis with embryo whole mounts was used to demonstrate that this protein is incorporated into punctate interblastomeric foci during compaction, consistent with its assembly into gap junction plaques. We conclude that connexin43 is one member of the connexin gene family whose zygotic expression is critical for preimplantation morphogenesis.