Synopsis. Electrical coupling, the passive spread of charge between cells, is usually char? acterized by microelectrode measurements. It can be mediated by gap junctions, cyto? plasmic bridges or extracellular space enclosed by specialized membranes. The morpho? logical basis affects the classes of molecules that can be transmitted by way of the coupling pathway. Conductance of gap junctions is controllable; it is decreased by a modest increase in intracellular H ions or a much larger increase in Ca ions. In amphibian blastomeres junctional conductance is reduced by transjunctional voltage. Gap junctions form and disappear non-specifically or specifically in different cases, but the origin and fate of junctional material are unknown. The common occurrence of electrical coupling in em? bryos has suggested a role in intercellular transfer of information, but direct evidence is lacking. A number of examples are known where gap junctions are found at specific times appropriate to suggest that a message is transmitted between the coupled cells. The message probably can be a distinct qualitative signal or a quantitative one resulting in a gradient among cells. Gap junctions are clearly absent at some sites of embryonic inter? action, but increasing evidence implicates an informational role at other sites.