Eleven neuropsychologically normal Parkinsonian patients were studied with [18F]6-fluoro-L-dopa and positron tomography. In all of the patients dopaminergic activity was reduced in the putamen on the side opposite to the major motor signs. The reduction was similar in tremulous and rigid patients. In contrast dopaminergic activity was normal in the caudate nuclei. It is argued that the putamen is mainly involved in the regulation of movement while the caudate nuclei assume a role in cognitive processes.