Common marmosets were given a subcutaneous injection of MPTP (1.25-2.5 mg/kg twice a week) for 5 or 10 consecutive months and were sacrificed after a survival time of 6 months or 15 days, respectively. The parkinsonian symptoms were not very marked at the time of sacrifice but there was a strong decrease of dopamine and, to a lesser extent, of its metabolites in the striatum and in some extrastriatal regions. There was also a profound loss of serotonin in the striatum and in all of the extrastriatal regions analyzed, which was still highly significant 6 months after discontinuation of MPTP treatment. The results suggest that the selected dosage schedule produces a widespread and lasting neuronal degeneration closely resembling the neurochemical pathology of Parkinson's disease.