A wide range of cognitive impairments can be observed in patients with Parkinson's disease. A close parallel exists between these deficits and those found following damage to prefrontal cortex. Anatomical evidence is reviewed which reveals a complex pattern of neuronal circuits connecting the frontal cortex and basal ganglia. All these circuits are in some way dependent upon dopamine, suggesting that changes in the levels of dopamine stimulation may alter performance on 'frontal' tests. To test this hypothesis, a group of patients with Parkinson's disease were assessed both on and off levodopa treatment, on a range of tests selected from the human and animal experimental literature as being sensitive to disruption of prefrontal cortex. A variable pattern of results was obtained. On one test, a measure of verbal fluency, patients were impaired, compared with normal controls, only when off levodopa. On two measures, associative conditional learning and subject- ordered pointing, patients were impaired only when on levodopa, while on the final measures, the Wisconsin Card Sorting Test, patients were impaired both on and off levodopa. Two mechanisms are discussed to explain these results, one based on the effects of dopamine depletion, and the other based on the adverse effects of dopamine overstimulation. The results suggest that different areas of prefrontal cortex are involved in the tasks employed, and that functional levels of dopamine in separate areas of cortex and caudate may differ crucially in Parkinson's disease.