Parkinsonian patients present an impairment of proprioceptor-guided movement that could imply abnormal processing in the frontal mesial cortex. To test this hypothesis, we compared neuronal response to joint displacement in the supplementary and presupplementary motor areas of two monkeys, before and after the progressive establishment of an 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonian syndrome. After MPTP administration, neurons were activated by the passive movement of numerous joints in various directions and no longer simply by one or two joints in one direction. This impairment of the focused selection of proprioceptive inputs, imputable to dopamine depletion, could impede motor planning and thus contribute to akinesia.