1. Intercellular communication between embryonic muscle fibres was examined in Drosophila melanogaster. 2. Injection of fluorescent dye revealed extensive coupling between muscle fibres which form a uniform communicating arrangement of cells without restriction at the segmental borders. 3. Dye transfer was blocked by octanol and membrane depolarization suggesting that it is mediated by gap junctions. 4. Double voltage-clamp experiments from cell pairs in situ showed that the ionic coupling is sensitive to the voltage difference between the cytoplasm and the extracellular space (transmembrane voltage, Vi-o) as well as between the cells (transjunctional voltage, Vj). 5. In steady-state conditions, the gap conductance (gj) was maximal for hyperpolarized Vi-o and decreased progressively to near zero as Vi-o became more positive than -50 mV. 6. Gap conductance decreased from a maximal value as Vj increased either in the positive or negative direction (by depolarizing or hyperpolarizing, respectively, one of the cells from a holding potential of -60 mV). In both cases, gj asymptotically approached a non-zero residual value which was different for negative and positive Vj (about 20% of the maximal conductance for negative transmembrane potentials and 10% for positive values). 7. Application of octanol (1 mM) resulted in an almost complete and reversible block of gj.
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