This study examined the effects of l‐dopa medication in patients with Parkinson’s disease on cortical and subcortical blood flow changes during two tasks known to involve frontostriatal circuitry. Eleven patients with Parkinson’s disease were scanned on two occasions, one ON l‐dopa medication and one OFF l‐dopa medication, during performance of the Tower of London planning task and a related test that emphasized aspects of spatial working memory. l‐dopa‐induced decreases were observed in the right dorsolateral prefrontal cortex during performance of both the planning and the spatial working memory tasks compared with the visuomotor control task. Conversely, l‐dopa‐induced blood flow increases were observed in the right occipital lobe during the memory task relative to the control task. Data from age‐matched healthy volunteers demonstrated that l‐dopa effectively normalized blood flow in these regions in the patient group. Moreover, a significant correlation was found between l‐dopa‐induced, planning related blood flow decreases in the right dorsolateral prefrontal cortex and l‐dopa‐induced changes in performance on the planning task. These data suggest that l‐dopa ameliorates high‐level cognitive deficits in Parkinson’s disease by inducing relative blood flow changes in the right dorsolateral prefrontal cortex.