This study describes the changes which occur in intra- and extracellular asymmetric acetylcholinesterase (AChE) forms in rat gracilis muscle during postnatal development. Initially (day 7) these forms (12.5 S and 16 S AChE) were evenly distributed along the muscle and only present intracellularly. With advancing age (days 7-28) they gradually became concentrated in endplate (vs non-endplate) muscle regions where a certain proportion of them was subsequently externalized. In contrast, no externalization was observed in the non-endplate regions. Our results support the view that AChE asymmetric forms are assembled within the muscle cell prior to their deposition on the extracellular synaptic compartment.