The expression and localization of the proline-rich membrane anchor (PRiMA), an anchoring protein of tetrameric globular form acetylcholines-terase (G4 AChE), were studied at vertebrate neuromuscular junctions. Both muscle and motor neuron contributed to this synaptic expression pattern. During the development of rat muscles, the expression of PRiMA and AChET and the enzymatic activity increased dramatically; however, the proportion of G4AChE decreased.