Previous studies showing downregulation of bloodbrain barrier (BBB) glucose-transporter activity in vivo in experimental diabetes have recently been questioned. Therefore, our investigations examined both the BBB glucose-transporter activity in experimental diabetes in vivo, with the use of the in situ internal carotid artery perfusion technique, and the microvessel glucose-transporter concentration, with quantitative Western immunoblots. These studies show that BBB glucose-transporter activity in vivo is decreased 44% in experimental diabetes (blood glucose concn 443 ± 12 mg/dl), parallel with a 44% decrease in cerebral blood flow, a normal brain blood volume, and a 75% prolongation of capillary transit time. The glucose-transporter concentration is decreased 77 ± 9% in microvessels isolated from diabetic rat brain compared with controls, as measured by quantitative Western blotting. These studies confirm the original observations that the BBB glucose transporter is downregulated in experimental diabetes, in association with a parallel decrease in cerebral blood flow. In addition, these studies, in conjunction with other recent experiments from this laboratory showing elevated glucose-transporter mRNA in diabetic rat brain capillaries, suggest the primary mechanism underlying the downregulation is a posttranscriptional inhibition of glucose-transporter mRNA translation.