Glutamate decarboxylase (GAD) is an enzyme that synthesizes gamma-aminobutyrate (GABA), a major inhibitory neurotransmitter in the central nervous system. Post-translational modification of GAD, such as N-terminal blockage, phosphorylation-dephosphorylation, and palmitoylation, is an important factor in the biological activity of GAD. In order to address the significance of post-translational events on GAD, we thought it crucial to obtain a non-recombinant form of GAD. In this study, we attempted to isolate GAD protein from the monkey brain, a model animal close to the human that has not been studied. Monkey brain was homogenized, fractionated with ammonium sulphate, and applied to a series of chromatographic steps, including hydrophobic, ion-exchange, and gel filtration. Purified GAD showed a single band on SDS-PAGE, and the enzyme was found to have a molecular weight of 61,000 and exhibited 1,100 nmol/min/mg of specific activity. It had an optimal pH of 7 and optimal thermal stability at 40 degrees C.