MICROELECTRODES have been inserted into both pre- and post-junctional elements of certain one-way synapses in the abdominal nerve-cord of the crayfish (Astacus fluviatilis). The pre-synaptic fibres are giant axons (lateral and medial) which run through the length of the cord. The post-fibre is, in each case, the largest of the motor axons which leave the ganglion in the third root. Fig. 1 shows an example of nerve-impulse transmission across such a synapse. The pre-junctional axon was stimulated with external electrodes; and its action potential, recorded intra-cellularly close to the synapse, is shown on the upper trace. The ensuing post-synaptic response, also recorded intracellularly from the junctional region, is shown on the lower beam. The magnitudes of the pre- and post-spikes were 92 and 70 mV., respectively. The post-spike was initiated when the synaptic potential exceeded about 20 mV. In most experiments the preparations were not in as good condition and only a synaptic potential was recorded from the post-fibre. It is difficult to assign a value to the delay between pre-spike and synaptic response. Both potentials seem to arise at about the same time but at different rates. When a test for antidromic transmission is made, it is found that a spike in the post-fibre usually gives rise to no detectable potential change in the pre-axon.