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1. The facilitation of neuromuscular transmission, which occurs during repetitive activation, was examined in the proximal accessory flexor muscle in walking legs of the lobster using electrophysiological techniques.

2. Post-synaptic potentials (p.s.p.s) in different muscle fibres facilitated to markedly different degrees. P.s.p.s in some fibres did not facilitate at all, while in others they increased in size by 20-30 times during stimulation at 20 Hz even though all the excitatory neuromuscular synapses are made by a single axon.

3. Stimulation of widely separated groups of synapses on any single muscle fibre evoked p.s.p.s with closely matched facilitation properties. Extracellular p.s.p.s recorded from single synaptic spots showed the same characteristics of facilitation as those of intracellular p.s.p.s in the same muscle fibre, suggesting that individual synaptic contacts on any single fibre are similar to each other.

4. Facilitation can be accounted for by an increase in the number of quanta released from the nerve terminals. There is no evidence for an increase in post-synaptic membrane sensitivity.

5. Low Ca solutions reduce transmitter release with comparatively little change in facilitation, while Cs solutions increase the size of p.s.p.s without increasing the amplitude of spontaneous miniature potentials. Thus, at poorly facilitating synapses it is unlikely that the absence of facilitation is caused by the saturation of some post-synaptic process.

6. It is concluded that the excitatory presynaptic nerve terminals on a single muscle fibre have matching facilitation characteristics. Some interaction between individual muscle fibres and their associated nerve endings may be required to establish or maintain this matching.