Functional magnetic resonance imaging (fMRI) is used to investigate the basal ganglia (BG)-cortex circuit using a rat model of Parkinson's disease (PD). The model involves a unilateral destruction of the right substantia nigra by intranigral injection of the dopaminergic neurotoxin 6-hydroxydopamine. Volume of cortical activity was measured by the blood oxygenation level-dependent contrast method while applying electrical forepaw stimulation. The main findings are the following. (i) Contrary to the predictions of the classic model but in line with recent experimental results (positron emission tomography, fMRI and electrophysiology), an increased cortical activity in the sensorimotor cortex of PD rats compared with sham-operated or normal rats was found. (ii) A diffuse neuronal activity at large cortical areas that were not related directly to the stimulation used, was observed. (iii) No difference was found between the lesion and the nonlesion hemispheres when the left or the right forepaw was stimulated; both cortices show significant overactivation of the sensorimotor cortices in addition to diffuse cortical activation. The last finding could be explained by either corticocortical connections or by bilateral BG-cortex connections. These finding suggest that the mutual influence of the two hemispheres is important in the pathophysiology of the BG-cortex circuit and might be crucial in predicting treatments.