To investigate the properties of the gamma-aminobutyric acid (GABA) synthesizing enzyme, glutamate decarboxylase (GAD), in the brain and the pancreatic islets of the rat, GABA concentration in the brain and the pancreatic islets was measured after administration of 3-mercaptopropionic acid (3-MP) at 25 mg/kg intraperitoneally. Sixty minutes after the administration of 3-MP, GABA concentration in the hypothalamus, the superior colliculus and the hippocampus of the brain decreased by 20-30% and in the pancreatic islets by 35%. The activities of GAD in the pancreatic islets and brain can be modified by a convulsant, in this case 3-MP. These results suggest the properties of GAD may be similar in the pancreatic islets and brain.