The action of tetrodotoxin on lumbar motoneurons of the cat was studied by means of intracellular recording. The local application of tetrodotoxin to the exposed cord surface effectively blocked impulse conduction in dorsal and ventral roots and intraspinal neurons. The threshold of motoneurons increased several-fold in the presence of the toxin without significant alterations in their resting membrane potential, membrane resistance and time constant. The recurrence frequency of spontaneous subthreshold potentials recorded from motoneurons was reduced, on the average, to about 20% of control values when tetrodotoxin was given. The toxin also caused a decrease in the mean amplitude of these potentials and virtually none were observed with amplitudes greater than 0. 4 mv. The effects of tetrodotoxin on cat spinal motoneurons are similar to those demonstrated on peripheral excitable tissues.