Five rhesus monkeys were trained to a 90% preoperative performance criterion on a color discrimination problem and also on a direct delayed response problem. Unilateral or bilateral caudate lesions were placed stereotaxically in 4 of the subjects after training. Caudate lesions produced no change in color discrimination ability, while delayed response ability was impaired. Unilateral lesions of the caudate resulted in a well defined partial loss of delayed response abilities, while a large bilateral lesion was followed by complete loss of delay ability. The control animal had no delay impairment following large bilateral frontal cortical ablation. Phenidylacetate and reserpine restored preoperative delayed response performance levels in animals with caudate lesions. Caudate lesions produced a difficulty in performing tasks where forced time delay was involved.