The gamma-aminobutyric acid (GABA) concentration of pancreatic islets in rats treated with streptozotocin (STZ) and of human insulinoma tissue was studied. Seven hours after the administration of 65 mg/kg body weight of STZ, a distinct increase in serum insulin concentration and at the same time a decrease in blood glucose level were seen. Twenty-four hours after the injection of STZ, however, the level of serum insulin decreased much, whereas that of blood glucose increased considerably. On the other hand, the GABA concentration of the islet was reduced dramatically to about one-tenth the control level after both 7 and 24 h. The histologic investigations of the islets revealed the destruction of B cells but no changes in A and D cells 7 and 24 h after the treatment of STZ. Nerve fibers and nerve endings in the islets were preserved intact all through the study. The GABA and insulin contents of the two cases of human insulinoma were determined. One insulinoma, which was compactly occupied with B cells according to its histologic features, contained a high concentration of GABA. The other tumor, having a rather sparse distribution of B cells in it as compared with the former case, possessed a lower concentration of GABA, but it was still high compared with that of its surrounding tissues. The present observations indicate that a large amount of GABA is available in the B cells of the pancreatic islets.