The endothelial cells of the mature cerebral microvessels, provided with barrier devices (blood-brain barrier, BBB), selectively express the glucose transporter isoform 1 (GLUT1). Presence and localization of the GLUT1 were studied by immunogold silver staining (IGSS) labelling on ultrathin sections of foetal human telencephalon tissue embedded in Lowicryl HM20 according to the progressive lowering of temperature (PLT) method. In the microvascular endothelial cells of the human telencephalon GLUT1 molecules are detected at the 12th gestational week and their expression is increased at the 18th week. In both ages, the transporter is mainly localized on the ablumenal and lateral endothelial cell membranes, and at 18 weeks a greater number of GLUT1 antigenic sites are also seen at the lumenal membrane. Our findings demonstrate both the expression and subcellular localization of GLUT1 be developmentally regulated and suggest an early functioning of the BBB-GLUT1 transporter in the developing human brain.