We have examined correlations between morphological and functional evidence of cell coupling between aggregates of beating embryonic heart cells and underlying layers. Synchronously beating aggregate-layer pairs were compared with asynchronous pairs. Intracellular microelectrode studies demonstrated that asynchronously beating aggregate-layers could not be induced to beat synchronously by electrical stimulation of the aggregate, whereas 86% of synchronous instances showed propagation of stimulating current pulses from aggregate to layer. By freeze fracture we have found significant differences both in the number and in the total area of gap junctions between the aggregate-layer interfaces of synchronous and asynchronous preparations. The data suggest that synchronous beating is a reliable functional indication of effective ionic coupling, and requires a certain area and number of gap junction/cell.