. Insulin stimulates glucose transport in adipocytes via the rapid redistribution of the GLUT! and GLUT4 glucose transporters from intracellular membrane compartments to the cell surface. Insulin sensitivity is dependent on the proper intracellular trafficking of the glucose transporters in the basal state. The bulk of insulin-sensitive transport in adipocytes appears to be due to the translocation of GLUT4, which is more efficiently sequestered inside the cell and is present in much greater abundance than GLUTI.The cell type and isoform specificity of GLUT4 intracellular targeting were investigated by examining the subcellular distribution of GLUTI and GLUT4 in cell types that are refractory to the effect of insulin on glucose transport. Rat GLUT4 was expressed in 3T3-LI fibroblasts and HepG2 hepatoma cells by DNAmediated transfection . Transfected 3T3-Ll fibroblasts over-expressing human GLUTI exhibited increased glucose transport, and laser confocal immunofluorescent imaging of GLUTI in these cells indicated that the protein was concentrated in the plasma membrane. In contrast, 3T3-Ll fibroblasts expressing GLUT4 exhibited no increase in transport activity, and confocal imaging demonstrated that this protein was targeted almost exclusively to cytoplasmic compartments . 3T3-Ll fibroblasts expressing GLUT4 were unresponsive to r.UCOSE, an essential nutrient for many mammalian cells, is transported across the plasma membrane via facilitative carrier proteins . Five glucose transporter isoforms have been described thus far (for reviews, see Gould and Bell, 1990;Mueckler, 1990). These are named GLUTI through GLUT5 in order of their identification by means of cDNA cloning. The five transporter isoforms are distinguished by their discrete tissue distributions, kinetic properties, and regulation . In most cell types, such as endothelial cells, hepatocytes, and parenchymal cells of the brain, glucose transport is probably not rate limiting for glu-