The anterior portion of the right cingulate cortex was ablated in four female stump-tailed macaques (Macaca speciosa). Two had concomitant right supplementary motor cortex lesions. One had the supplementary lesion preceding the cingulectomy. The fourth had the cingulate lesion followed by supplementary ablation. Cingulectomized monkeys showed the contralateral neglect syndrome. Supplementary motor cortex ablation did not alter the performance of any animal on the tactile task. Lesions in the inferior parietal lobule, around the superior temporal sulcus, and of the frontal arcuate sulcus have produced neglect. These areas project to the cingulate gyrus. The cingulate gyrus in turn projects to the mesencephalic reticular formation. A cingulate gyrus lesion therefore interrupts this corticolimbic reticular activating pathway. We feel the neglect syndrome produced by this lesion is caused by a unilateral defect in the alerting response to sensory stimuli. © 1973 American Academy of Neurology.