Male white mice were injected with N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) 40 mg/kg. After 2 h, 24 h, 2, 4 and 8 days 0.25 mCi [5-3H]uridine was injected i.v. and 1 h later the animals were sacrificed. In microautoradiograms the number of grains over the nerve cell nuclei was counted in the substantia nigra, the cerebral cortex, the hippocampus and the neostriatum. In the substantia nigra there was a significant increase in grain number after 2, 4 and 8 days. In a group of animals injected 3 days with MPTP and sacrificed 2 days after the last injection the same increase was found. In the other brain regions only an insignificant increase in grain counts was found. In a special group of animals the total radioactivity and distribution of label on metabolites in blood, brain and liver was determined. No significant changes were found.