Single shock stimulation of the optic nerve stump of one side induces an increase in excitability of the central terminals of the contralateral optic nerve at the level of the lateral geniculate body and superior colliculus. Concurrently with the increase in excitability a slow negative potential is also recorded from the optic nerve. Both phenomena suggest an active process of depolarization occurring at the level of the terminals and spreading electrotonically to the optic nerve. Such process of depolarization is considered as the basic mechanism for presynaptic inhibition [4].