The formation of nerve-muscle junctions in monolayer cultures of embryonic muscle and spinal cord cells is described. Muscle-forming cells (myoblasts) from leg muscles of 12-day chick embryos were separated with trypsin and cultured on a collagen substrate for two days. Suspensions of ventral spinal cord cells from six-day chick embryos were then plated over the differentiating muscle cells. The cultures were subsequently examined by electron microscopy and in silver-stained light microscopic preparations. After 10-12 days in culture, irregular thickenings were observed along the nerve cell processes in contact with the muscle fibers which by that time had undergone advanced differentiation (myogenesis). By electron microscopy it was demonstrated that many of the nerve-muscle contacts had the characteristics of a synapse. Some aspects of the fine structure of these junctions are described. The possibilities raised by these findings with respect to innervation mechanisms and specificities in nerve-muscle interactions are briefly discussed.