Patients with Parkinson's disease (PD) have trouble programming two separate motor acts concurrently. We tested the hypotheses that (1) PD patients may also have difficulty processing two cognitive tasks simultaneously, and (2) the expected deficit may be related to the striatal dopaminergic depletion. We used auditory and visual choice reaction time (CRT) tasks, presented either separately or concurrently, and compared the performance of three groups of PD patients: a group of patients under their usual dose of levodopa ("standard"); a group assessed both at the time of maximal clinical benefit ("on" state) and at the time of minimal clinical benefit (treatment withdrawn for about 18 hours; "off" state); and a group of recently diagnosed untreated patients ("de novo"). Compared with controls, standard and "on" state patients had a normal performance for both separate and concurrent CRT tasks. In contrast, "off" state and de novo patients had a normal performance in the separate CRT tasks but significant deficits in the concurrent CRT tasks. These results suggest that adequate dopaminergic transmission is necessary for concurrent processing of cognitive information and that the striatum integrates the sensorimotor information required to program cognitive acts.