Inhibitory action of iontophoretically applied GABA was examined on neurons in the visual layer of the rat superior colliculus (SC). Spontaneous discharges of all neurons tested were readily abolished by GABA ejected with currents less than 25 nA. In some cells the discharges evoked by near threshold electrical stimulation of the optic nerve or those evoked by a spot of light moving across receptive fields were suppressed by the same dose of GABA as that required to abolish the spontaneous discharge. However, in other cells the evoked discharges were much more resistant to GABA than the spontaneous activity. GABA sensitivity of the evoked activities was examined on various classes of SC cells which were identified by their recording depth, response latency to electrical stimulation of the optic chiasm and other properties. SC cells of the visual layer were classified into 8 types: classes Ia and Ib in the most superficial layer (N3 zone), class II in the thin layer below the N3 (N2 zone) and classes IIIa, IIIb, IVb and IVc in the deepest layer below the N2 (N1 zone). Effects of GABA upon these cell classes are summarized as follows; (1) Ia and IVb cells were readily suppressed by GABA, (2) Ib and II and most of IIIa and IVc cells were GABA-insensitive, and (3) GABA sensitivity varied from cell to cell in classes IIIb and IVa.