1. A single impulse in any one of the central giant fibres of the crayfish is sufficient to evoke a full escape response.

2. Following such a single impulse a search was made for inhibitory processes of similar duration to the driving movement of the escape response.

3. There is no inhibition of flexor motoneurones or muscles to prevent response to impulses in the central giant axons during the escape response. However, following an impulse in any central giant, intracellular recording showed that there is inhibition of excitatory input to the lateral giants in the abdomen. This inhibition suppresses impulse generation for the duration of the escape response.

4. The inhibition coincides with slow, depolarizing potentials in the lateral giants. These have an equilibrium potential between the normal resting potential and the threshold for spike initiation in the lateral giants. During these slow potentials there is a postsynaptic resistance decrease coinciding very closely in time course with the inhibition of excitatory input. The slow potentials are therefore identified as IPSPs (inhibitory postsynaptic potentials) because of their close association with a postsynaptic inhibitory process. This conclusion is endorsed: (a) by the absence of similar slow potentials in the abdominal medial giants which have no excitatory input at this location, and (b) by the diminution of the slow potentials by picrotoxin, a drug known to block inhibition at many crustacean synapses.

5. When evoked repetitively, even at low frequencies like 0.25 per sec, the IPSPs decrease in amplitude. No other ‘after effects’ of repeated activity were found.

6. Attempts to localize the inhibitory synapses are frustrated by the large space constant of the lateral giants. However, the evidence is compatible with the notion that inhibition originates within each abdominal ganglion. There is occlusion and crossed response decrement between the central giant axons evoking lateral giant inhibition. This suggests that the different presynaptic fibres excite some common inhibitory pathway in each ganglion. Further experiments showed that pathways producing inhibition in one ganglion can be excited in others. Interneuronal arrangements to explain properties of the inhibitory pathways are discussed.

7. Two functions are suggested for the recurrent inhibition in the crayfish lateral giants. First, it may limit the number of impulses that are evoked by a single afferent excitatory volley. Secondly, it may coordinate successive escape responses by suppressing impulse generation in the lateral giants during such responses.

Supported by a Cardiovascular Traineeship from the U.S. Public Health Service.