1. The dissection of a small length of a giant axon between the 5th and the 6th ganglia of the abdominal nerve cord of the cockroach Periplaneta americana allows extracellular recordings of excitatory post-synaptic potentials (EPSPs) to be made in particularly good conditions.
2. ‘Spontaneous’ or ‘induced’ EPSPs are smaller and slower than those recorded with conventional micro-electrodes.
3. Post-synaptic action potentials are very similar to, or even larger than, those recorded by means of intracellular electrodes.
4. The low impedance of the recording system, together with its stability, allows numerous experiments to be made which were nearly impossible with the micro-electrode technique.
5. It is suggested that this technique might be used for other post-synaptic structures.