1. The development of AChE at ectopic neuromuscular junctions forming between a transplanted foreign nerve (the superficial fibular nerve) and the denervated soleus muscle has been studied in adult rats. 2. Junctional AChE activity began to appear in the vicinity of the fibular nerve sprouts 6-7 days after section of the soleus nerve and 3-4 days after the onset of transmission. 3. No histochemically detectable AChE appeared when the fibular nerve was cut 0-4 days after the soleus nerve had been cut. 4. Direct electrical stimulation of the denervated soleus muscle caused plaques of true AChE, as determined by inhibitor studies, to appear in muscles where the fibular nerve had been cut 2-4 days after the soleus nerve but not in muscles where the two nerves had been cut at the same time. The plaques appeared only in the vicinity of fibular nerve sprouts and coincided with newly formed but stable peaks of ACh sensitivity. Local application of Neostigmine prolonged and increased the depolarising response evoked by pulses of ACh at these sites. 5. In muscles where the fibular nerve was intact the AChE plaques changed gradually over a few weeks from an immature appearance to a mature appearance characteristic of normal end-plates. In stimulated muscles where the fibular nerve had been cut the plaques stained intensely but remained morphologically immature. 6. We conclude (1) that muscle activity is important for the appearance of AChE at developing neuromuscular junctions and (2) that AChE accumulates only at sites on the muscle surface where the nerve fibres have left a 'trace' upon contact with the muscle fibres. These traces form quickly and persist after nerve-muscle interaction of as little as 2 days. The muscle appears as a major source of junctional AChE since stimulation of the muscle induces intense AChE activity in muscles where the nerve has degenerated.