Thiamine pyrophosphatase (TPPase) activity was localized in the terminal arborization of the motor end plates both in rat and frog striated muscles. Electron microscopically the end product of the enzyme was seen in synaptic vesicles. That TPPase was axonally transported was indicated by its localization following experimental ligation of peripheral nerves. It is thus possible that TPPase may play a role in the synthesis of the synaptic mediator substance of the neuromuscular junction.