Numerous abnormalities of motor function have been identified in Parkinson's disease and functional imaging methods have given us a better understanding of underlying mechanisms. An underactivity of some frontal areas, in particular the supplementary motor area and the dorsolateral-prefrontal cortex, has been found in several studies. The activity of these areas increases when motor function improves following administration of dopaminergic drugs or functional neurosurgery. The dysfunction of the basal ganglia-thalamo-cortical loops in Parkinson's disease and in particular the excess of inhibition of the thalamo-cortical pathway by the internal pallidum is likely to be involved in the abnormal activity of the cortical areas. An excess of activation has been demonstrated in other areas and is probably compensatory.