NCAM in its high polysialic acid (PSA) form is expressed on chick hindlimb motoneurons during their growth, and then decreases at about the time that synaptogenesis is completed. In order to characterize this regulation at the cell and molecular level, the present studies use the chick ciliary ganglion (CG) system, which constitutes a homogeneous and developmentally synchronized population of motoneurons that can be used for in vitro studies. Levels of PSA in the CG were evaluated both by SDS-PAGE immunoblot analysis of total NCAM and by pulse radiolabeling of newly synthesized NCAM. Up- and downregulation of PSA expression on newly synthesized NCAM in the CG was found to be closely correlated with in vivo innervation and synaptogenesis, respectively. Moreover, the downregulation observed at synaptogenesis was prevented by in vivo blockade of neuromuscular activity with alpha- bungarotoxin. The developmental regulation of PSA expression was found to coincide precisely with an increase and decrease in levels of specific polysialyltransferase activity. By contrast, the expression of the VASE exon in NCAM, which in CNS is temporally correlated with PSA downregulation, was not expressed in the CG. Cocultures of CG neurons with myotubes were used to provide direct evidence that neuron-muscle interaction can cause a specific downregulation of both neuronal PSA and polysialyltransferase activity.