The mRNA levels encoding neuropeptides were measured in the caudate nucleus, putamen and nucleus accumbens of common marmosets exposed to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine pyridine (MPTP). Motor deficits induced by MPTP treatment were characterized by akinesia, postural abnormalities and rigidity. Seven days after MPTP treatment, there was a marked increase in levels of enkephalin mRNA in the caudate nucleus and putamen. In contrast, the hybridization signal for substance P mRNA was reduced. Alterations in the mRNA encoding neuropeptides were similar but less extensive in marmosets at 18-50 months following MPTP treatment. No significant changes in enkephalin or substance P mRNA in the nucleus accumbens were observed at either time. Treatment with L-DOPA plus carbidopa for 4 weeks reversed MPTP-induce motor deficits and other behavioural abnormalities. The decrease in substance P mRNA in the striatum of MPTP-treated animals was reversed by L-DOPA treatment and reached levels above those found in normal animals. In contrast, the increase in enkephalin mRNA in marmosets treated with MPTP was not altered by L-DOPA treatment. In the nucleus accumbens the levels of peptide mRNA were not affected by L-DOPA treatment. Loss of nigral dopamine cells in a primate species causes opposing alterations in the expression of enkephalin and substance P mRNA in the caudate nucleus and putamen. No changes were observed in the nucleus accumbens, which reflects the resistance of the mesolimbic neurons to MPTP toxicity. While the decrease in substance P mRNA was reversed by L-DOPA treatment, the increase in enkephalin mRNA was not. This may partly indicate the greater effect of L-DOPA on the direct GABA pathway compared to the indirect output pathway from the striatum.