The mRNA levels encoding for the enzyme glutamate decarboxylase (GAD67) were measured by computerized image analysis after in situ hybridization histochemistry and radioautography in the striatum and pallidum of normal squirrel monkeys (Saimiri sciureus), or after treatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP). All MPTP-injected monkeys exhibited profound motor deficits including akinesia. The dopaminergic innervation, as visualized and quantified on x-ray films after 3H-mazindol binding on tissue sections, was uniformly lost throughout the striatum of MPTP- treated monkeys. Brain sections processed with a probe synthesized from a feline or human GAD67 cDNA exhibited intense radioautographic labeling throughout the striatum. When measured on x-ray films, the intensity of GAD67 mRNA labeling was increased in the striatum of MPTP- treated versus control monkeys. Increased labeling reached statistical significance in the dorsolateral sector of the rostral putamen and throughout the putamen and the caudate at the caudal, postcommissural, level. Analysis of emulsion radioautographs demonstrated that the increase in GAD67 mRNA labeling in MPTP-treated monkeys occurred in individual neurons of the striatum. In the external and internal segments of the pallidum, numerous neurons labeled with the GAD67 cRNA probe were visualized on emulsion radioautographs. The intensity of GAD67 mRNA labeling in single neurons of both pallidal segments was increased in MPTP-treated versus control monkeys. Construction of the histograms of frequency distribution of labeling indicated that this increase occurred in a majority of labeled neurons. The present study demonstrates that GAD67 mRNA levels are significantly altered in the striatum and pallidum of parkinsonian monkeys. The preferential increase of GAD67 mRNA labeling in the dorsolateral putamen, which receives afferents from the sensorimotor cortex, provides further evidence of the involvement of GABAergic transmission in the expression of the motor deficits elicited after MPTP. In addition, increased GAD67 mRNA levels in the internal segment of the pallidum support the hypothesis of an increased activity of GABAergic neurons in the output structures of the basal ganglia in parkinsonism.