Possible mechanisms by which neurons can affect the differentiated states of mononucleated cells in the early chick embryo leg skeletal myogenic lineage are being investigated. Under conditions of co-culture known to promote inductive interactions between embryonic neurons and leg cells, spinal cord neurons and mononucleated leg cells are coupled by gap junctions as shown by cell-to-cell passage of Lucifer yellow and hyperpolarizing or depolarizing currents. Co-cultures composed of ciliary ganglion neurons and clones of leg cells showed that the ability to form gap junctions with neurons is a differentiated function of myoblasts. Dye-coupling with neurons was found at high frequency only when the partner cell was a myoblast of a differentiating muscle clone. Myoblasts in undifferentiated clones formed gap junctions with each other but not with neurons. The nature of the junction-forming neuron is not restricted since neurons from spinal cord, ciliary ganglion and dorsal root ganglion all readily form junctions with myoblasts.