Unilateral dopamine deficiency of the basal ganglia produced a profound impairment of visual search. Dopaminergic innervation of the monkey striatum was deprived unilaterally by infusing 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) into the caudate nucleus. The monkeys were shown a mirror in which they saw their own image and surroundings. Compared with the concentric and symmetrical distribution of visual search before MPTP infusion, the gaze after MPTP infusion was confined and sometimes frozen in peripheral regions in the hemispace ipsilateral to the infusion; the contralateral side was largely neglected. An experiment using a choice saccade task indicated that the contralateral visual signal, which alone was processed normally, was suppressed by the ipsilateral signal.