Discrete unilateral mesencephalic reticular formation lesions were made in four female stump-tailed macacques (Macaca speciosa). Control lesions of identical size were placed in the left medial dorsal thalamic nucleus, left ventral posterolateral thalamic nucleus, and left pontine nucleus. Experimental subjects showed profound tactile, visual and auditory neglect, in addition to ipsilateral hemispheric slowing on EEG. Controls did not demonstrate either phenomenon. This study suggests that neglect results from any lesion along the corticolimbic-reticular activating loop, and that the basis of neglect is a unilateral defect in the alerting response to sensory stimuli because of a disruption of this loop. © 1974 American Academy of Neurology.