We studied the involvement of γ-aminobutyric acid (GABA) as a neurotransmitter in the inhibition of the cat's superior colliculus (SC) that is one of the important visual center. Iontophoretical applications of GABA and the antagonist bicuculline were preformed to the SC neurons in anesthetized cat and the effects on the neuron activities were examined. Besides the distribution, of the neurons which have GABA receptors was investigated immunocytochemi cally using an antiserum raised againist bovine serum albumin-conjugated GABA. The results suggested that GABA at least involved in the inhibition mediated through GABA_A, receptors.