The aim of this study was to specify the frontal type dysfunction widely reported in Parkinson's disease (PD) early in the course of the disease and before dopaminergic therapy. Seventeen "de novo" PD patients and 17 healthy control subjects performed modified versions of the Stroop word-color test and the Brown Peterson paradigm. A dissociation between results on the two tasks was observed in PD patients. They had difficulties in inhibiting a strong habitual response and establishing a new, better adapted pattern of response; but they performed as well as controls in a dual-task paradigm requiring correct allocation of the processing resources of working memory. Early in the course of the disease, untreated PD patients suffer from dysfunction of the supervisory attentional system. However, the present findings suggest that this system is not a single unit but rather could be composed of multiple subsystems whose sensitivity depends on the origin of frontal dysfunction. Indeed, only a few of these subsystems seemed to be impaired in de novo PD patients. It can be hypothesized that those involved in the phenomena of adaptation and consolidation of currently appropriate responses depend on the dorsolateral prefrontal loop, which is affected by the dopaminergic innervation of the caudate nucleus.