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The basal ganglia, especially the ventral striatum, have been implicated in control of action based on motivation1,2,3,4. A prevalent view is that nigrostriatal dopaminergic neurons carry reinforcement signals to modulate the cortico-striatal signal transmissions5,6,7. However, it is still unknown how such reinforcement signals affect the output of the striatum in relation to behavior. To answer this question, we devised a memory-guided saccade task in which only one out of four directions was rewarded, and examined single cell activity in the caudate nucleus. We found that visual or memory-related responses of presumed projection neurons in the caudate were frequently modulated by expectation of reward, either as an enhancement or as a reduction of response. The cell's preferred direction often changed with the change in the rewarded direction, implying a short-term synaptic plasticity. The modulation of caudate cell activity was correlated with changes in saccade parameters. Our results suggest that the caudate contributes to the determination of oculomotor outputs by affiliating motivational values to visual information. METHODS INTRODUCTION We used two male Japanese monkeys (Macaca fuscata). Under general anesthesia, we implanted a head holder, chambers for unit recording, and a scleral search coil21. The monkeys were trained to perform saccade tasks, especially a memory-guided saccade task27. Eye movements were recorded using the search coil method. We recorded extracellular spike activity of presumed projection neurons which showed very low spontaneous activity28, but not of presumed interneurons which showed irregular tonic discharge29. For each cell that showed visual or memory-related responses, we used a set of four target locations with the same eccentricity that were arranged in either normal or oblique angles, depending on the cell's receptive field. The recording sites were verified using MRI (Hitachi, AIRIS, 0.3T). The monkeys performed the memory-guided saccade task in two different reward conditions: alldirections-rewarded condition (ADR) and onedirection-rewarded condition (1DR). For every caudate cell recorded, we required the monkeys to perform one block of ADR and four blocks of 1DR (i.e., four different rewarded directions).