The ability of gap junction antibodies to block dye transfer and electrical coupling was examined in the compacted 8-cell mouse zygote. In control zygotes, Lucifer yellow injected into 1 cell transferred to the rest of the embryo. When antibodies raised against the major protein extracted from gap junctions were co-injected with Lucifer yellow, dye transfer failed in 86% of the zygotes tested and electrical coupling was almost completely inhibited. Subsequently, the antibody-containing cells were extruded. When the antibodies were injected into 1 cell at the 2-cell stage, 82% of the zygotes divided normally to the 8-cell stage. Cells containing gap junction antibodies were uncompacted, but continued to divide. We conclude that these antibodies inhibit gap junctional communication in the early mouse zygote and that communication through gap junctions may be involved in the maintenance of compaction.