1. An axon which, in addition to the single excitatory axon, innervates the anterior coxal adductor muscles of locusts and grasshoppers causes peripheral effects, some of which are comparable to those of crustacean peripheral inhibitory axons. Tetanus tension is reduced by the axon.

2. The axon was termed the inhibitory-conditioning axon (I-C). Resting tension was reduced in a few preparations by the action of the axon, and the relaxation rate was markedly increased.

3. In some preparations, however, the axon caused a slight contraction of the muscle, and paradoxically enhanced twitch tension.

4. It was found by intracellular recording that in some muscle fibres the electrical response to I-C is polarizing, but in others it is depolarizing.

5. A few (but variable number) of the muscle fibres in each preparation showed slow, summating, after-depolarizations following termination of the excitatory junctional potentials. This is a new phenomenon, and suggests that these fibres, or their neuromuscular junctions, have special properties.

6. A study was made of the reflex and spontaneous discharges of the excitatory and I-C axons in the nearly intact animal, and in preparations, in an attempt to determine the function of I-C.

7. No evidence was found that I-C is actually used as a peripheral inhibitor and its natural function remains enigmatical.

Supported by research grant NSF GB3160.