The activity of glutamic acid decarboxylase (GAD), the enzyme involved in formation of the inhibitory neurotransmitter γ-aminobutyric acid (GABA), was studied in autopsy brain samples from six parkinsonian patients and 13 controls. There was relatively good postmortem stability of GAD in deep-frozen brain samples over a 3-week period. The activity of GAD was significantly reduced in brain samples of patients with Parkinson’s disease, being about 50% of that in controls. Moreover, levodopa treatment showed a tendency to increase the activity of GAD. The results suggest the involvement of GABA neurons in Parkinson’s disease.