IN vertebrate skeletal muscles, besides a strong acetylcholinesterase activity at motor end-plates, a weaker but well-defined activity is found at muscle-tendon junctions1,2. Whereas the functional role of acetylcholinesterase at the end-plates seems to be well established, both the origin and function of the enzyme at muscle-tendon junctions remain obscure. The question was often raised whether the concentration of the enzyme at these sites could not be in some relation to the motor or sensory innervation of the muscle. However, acetylcholinesterase activity at muscle-tendon junctions appears to be unaltered after nerve section in adult animals3. On the other hand, in tissue culture no concentration of the enzyme develops either in the middle part or at the ends of muscle fibres4, and it has been suggested that this might be due to the absence of nerves5.