A high concentration of gamma-aminobutyric acid (GABA) and a high activity of glutamate decarboxylase (GAD), enzyme for GABA synthesis, were demonstrated in the rat pancreatic islets using a recently developed assay method combining the GABase system with the enzymatic NADP-NADPH cyclin method. Both GABA concentration and GAD activity in the pancreatic islets were comparable to those in the central nervous system (CNS). An insulinoma consisting mainly of B cells possessed a high concentration of GABA and a high activity of GAD. On the other hand, in the pancreatic islets of rats after treatment with streptozotocin, a specific attacker of B cells, GABA was negligible in amount. The occurrence of GABA in the pancreatic islets, at least B cells, apparently supports the view of Fujita that the islet cells are paraneurons. Presence of exogenous GABA does not influence the glucose- and amino acid-induced insulin release. The uptake of 3H-GABA into the pancreatic islet was small in amount as compared with that into the brain tissue such as the substantia nigra. These observations suggest that the different compartmentation of GABA in the islets from that in the CNS.