The study of the formation of synaptic circuits within the spinal cord has been hampered by the absence of accessible experimental preparations and suitable techniques. Recently, two new preparations have been described for following spinal development in vitro: slices of embryonic rat spinal cord co-cultured with sensory ganglia and myotubes, and cultures of the entire central nervous system of neonatal opossums. Optical imaging techniques have revealed spontaneous synchronous increases of Ca2+ levels in groups of spinal neurons. The widespread electrical coupling among developing spinal neurons, which may contribute to these synchronous bursts, becomes highly restricted as development proceeds. Both phenomena may play central roles in the formation of specific patterns of synaptic connections.