Multicompartment nerve-muscle co-cultures are used to study activity effects on synapse development and stabilization. Functional nerve-muscle synapses are identified by co-localization of activity-dependent uptake of FM1-43 with sites of alpha-bungarotoxin binding. Expression and distribution of Na+K+-ATPase isoforms and N-type calcium channel subunits appear to be regulated developmentally in spinal cord cell cultures.