Effects of strychnine on spinal motoneurons of the toad were observed with intracellular electrodes. The action of the drug was classified into three stages depending on the dose of the drug or the period of time elapsed after its injection.In the 1st stage, it was found that strychnine depressed the inhibitory process and generated a train ot spikes by a single orthodromic stimulation.In the second stage an orthodromic volley produced a long-lasting depolarization with superimposed spikes which became successively smaller in amplitude corresponding to the degree of depolarization. In the third stage the motoneuron soma spontaneously presented a periodic discharge accompanied by a large depolarization wave. This periodic depolarization wave was not also negated by superimposed spikes. The site of action of strychnine was discussed with the conclusion that the primary action of the drug is to depress the inhibitory process in the spinal cord and that the periodic discharge of motoneurons in strychnine tetanus originates in presynaptic elements.