It is unclear how motivation leads to improved motor performance. Here we test the hypothesis that motivation interacts with behavioural performance in the basal ganglia. We recorded trial-to-trial performance in a bimanual motor task in 10 patients with Parkinson's disease with electrodes chronically implanted in the subthalamic nucleus for deep brain stimulation. Motivation-associated improvements in trial-to-trial performance were contrasted with and without stimulation at high frequency. Motivation and stimulation improved trial-to-trial performance, but the effect of motivation was halved during stimulation. We conclude that the subthalamic area is mechanistically important in those processes linking motivation to improvement in motor performance. This finding may be relevant to some of the cognitive and emotional changes associated with bilateral subthalamic stimulation.