Primates have the remarkable ability to rapidly adjust or modify associations between visual cues and specific motor responses. Whereas little is known as to how such adjustments in behavioral policy are implemented, recent learning models suggest that the anterior striatum is optimally positioned to have a role in this process. We recorded from single units and delivered microstimulation in the striatum of rhesus monkeys performing an associative learning task. Caudate activity during reinforcement was closely correlated with the rate of learning and peaked during the steepest portion of the learning curve when new associations were being acquired. Moreover, delivering microstimulation in the caudate during the reinforcement period significantly increased the rate of learning without altering the monkeys' ultimate performance. These findings suggest that the caudate is responsible for implementing selective adjustments to the 'associative weights' between sensory cues and motor responses during learning, thus enhancing the likelihood of selecting profitable actions.