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The GLUT-1 isoform of the glucose transporter is commonly accepted as a reliable molecular marker of blood-brain barrier endothelia in neural vasculature organized in a three-dimensional network of single vessels. The brain of the lizard Podarcis sicula is characterized by a vascular architecture based on a pattern of paired vessels. The presence and distribution of GLUT-1 were studied in adult lizards using both light and transmission electron microscopic techniques. Immunoperoxidase histochemistry was applied to sections from paraffin-embedded brain using gold-conjugated secondary antibodies to localize this antigen on ultrathin sections. The transverse sectioned pairs of vessels did not show the same size and, in particular, the two elements of the same pair often differed in their diameters. Light microscopy revealed immunopositivity in both parenchymal and meningeal vessels. In each transverse-sectioned vascular pair, one element was intensely labelled, and the adjacent one showed only slight or negligible reaction. Colloidal gold particles were restricted to endothelial cells, showing an asymmetric labelling pattern, which was always characterized by markedly higher density of immunolabelling of the abluminal rather than the luminal plasmalemma. Moreover, in every vascular pair, one profile had lower amounts of scantier labelling by gold particles than the adjacent element. This pattern indicates functional differences between the adjacent vascular limbs regarding glucose transport.