1. The nature of the transmitter substance released at the lateral-line efferent synapses was investigated by histochemical techniques in Xenopus and Acerina and by pharmacological methods in Xenopus.
2. The bases of lateral-line hair-cells and fine fibres in the lateral-line nerves reacted positively for acetylcholinesterase in Acerina and to a lesser extent in Xenopus.
3. Acetylcholine (10-6M, and acetyl-β-methyl choline (5 x 10-6M), which has a muscarinic action, caused strong reversible inhibition of afferent impulses when pipetted on to the undersides of lateral-line stitches. Carbachol (5 x 10-4 to 5 x 10-5M) caused a smaller reversible inhibition of spontaneous afferent impulses, but other nicotinic substances (propionylcholine and buterylcholine) had no effect.
4. Physostigmine (5 x 10-5M) prolonged inhibition of afferent impulses by electrical stimulation of efferent fibres, but atropine (5 x 10-6M) blocked it.
5. Calcium and magnesium interact at the efferent synapses in a way similar to that found at the amphibian neuromuscular junction.
6. Arguments are put forward to support the hypothesis that acetylcholine is released at lateral-line efferent synapses.