We have studied the regional cerebral blood flow (rCBF) changes induced by the execution of a finger-to-thumb opposition motor task in the supplementary and primary motor cortex of two groups of parkinsonian patients on L-dopa medication, the first one without L-dopa induced dyskinesia (n = 23) and the other with moderate peak-dose dyskinesia (n = 15), and of a group of 14 normal subjects. Single photon emission tomography with i.v. 133Xe was used to measure the rCBF changes. The dyskinetic parkinsonian patients exhibited a pattern of response which was markedly different from those of the normal subjects and non-dyskinetic parkinsonian patients, with a significant overactivation in the supplementary motor area and the ipsi- and contralateral primary motor areas. These results are compatible with the hypothesis that an hyperkinetic abnormal involuntary movement, like L-dopa-induced peak dose dyskinesia, is due to a disinhibition of the primary and associated motor cortex secondary to an excessive outflow of the pallidothalamocortical motor loop.