Intracellular microelectrodes were used to study a cholinergic synapse between two identified neurones: the lateral filiform hair sensory neurone (LFHSN) and giant interneurone 3 (GI 3) in the terminal ganglion of the first-instar cockroach Periplaneta americana. The presynaptic neurone (LFHSN) was impaled in a region of the axon which forms large numbers of output synapses onto GI 3. Intracellular injection of tetraethylammonium (TEA+) into LFHSN blocked LFHSN-GI 3 synaptic transmission. Injection of TEA+ and either acetylcholine (ACh) or choline into the axon preserved synaptic transmission. TEA+ may compete with choline at an intracellular site involved in the maintenance of releaseable ACh.