1. The effect of tetraethylammonium (TEA) ions on toad's spinal motoneurons was studied with intracellular microelectrodes.
2. The falling phase of the action potential was markedly prolonged after TEA application. The time-course of this phase was unstable and liable to change by electrotonus and temperature.
3. EPSP was first increased after TEA. In some motoneurons, spontaneous generation of action potentials was not infrequent. Single orthodromic stimuli caused violent irregular discharges from these motoneurons.
4. The action potential generated antidromically had much longer falling phase than that generated orthodromically. Early termination of the falling phase could be brought about by superimposed PSP's, when an antidromic stimulation was followed by an orthodromic stimulation at suitable time intervals.
5. The PSP's responsible for this abolishment of action potential are considered to be of inhibitory nature (IPSP) and the mechanism of the abolishment was discussed.
6. At later stages of TEA application, single shocks given to a dorsal root failed eventually to fire the motoneurons. This was caused either by attenuated PSP's or by hyperpolarizing PSP's which then became predominant.