Postsynaptic potentials (PSPs) were recorded on the surface of the rat in the superior colliculus (SC) after electrical stimulation of the optic nerve. Long-term potentiation (LTP) was elicited in the PSP by tetanic stimulation of the optic nerve either when the ipsilateral visual cortex was removed or when a GABA antagonist, picrotoxin, was administered before the tetanic stimulation. These results suggest that the corticotectal tract may inhibit the appearance of LTP via the GABAergic interneurons within the superficial gray layer (SGL).