ALTHOUGH the synaptic transmission across the last abdominal ganglion of the cockroach has been studied by several workers1–3, the knowledge obtained so far seems to be less complete than for synapses in vertebrate animals. In the present investigation electrical phenomena associated with the synaptic transmission across this ganglion were analysed with special reference to the nature of the synaptic transmitter. The cereal nerve–nerve cord preparation was isolated4, and postsynaptic responses were led off from the origin of the connectives from the last abdominal ganglion.