Previous studies revealed that rat islets express the GLUT2-liver facilitative glucose transporter isoform, a glucose carrier with a low affinity for glucose but a high capacity for glucose transport. These studies indicated the presence of a second glucose transporter in rat islets; however, they did not indicate to which of the five known facilitative glucose transporters it corresponded. In this study, we isolated RNA from rat islets of Langerhans and confirmed the presence of GLUT2 mRNA. In addition, we present data indicating that the second isoform expressed in islets is the GLUT1-erythrocyte isoform. The effect of culturing islets in 5.5, 8.3, or 11.1 mM glucose on the levels of GLUT1 and GLUT2 mRNA also was examined. The levels of GLUT1 and GLUT2 mRNA were two- and threefold higher, respectively, in islets cultured for 24 h in 11.1 mM glucose compared with those incubated in the presence of 5.5 mM glucose. Therefore, the previously observed increase in GLUT2 mRNA levels in the islets of rats made hyperglycemic by chronic infusion of glucose can be mimicked in vitro, implying that glucose regulates GLUT2 mRNA expression.