Glucose is transported from blood to brain tissue by facilitated diffusion of limited capacity. The regional variation of the glucose transport capacity is not known in detail because methods used previously have been too gross to detect fine regional differences. Therefore, it is not known to what extent the glucose transport capacity varies in proportion to regional blood flow and metabolic rate. To resolve this question, we used double-tracer, dual-label autoradiography to measure blood-brain glucose clearance and blood flow simultaneously in hypo-, normal, and hyperglycemic rats. From the values of glucose clearance and blood flow at various plasma glucose levels, we calculated the affinity constant and maximal transport capacity of the glucose transport system. The transport capacity (Tmax) varied in approximate proportion to the blood flow and, by inference, the metabolic rate. In contrast, the affinity constant (Kt) did not vary systematically between the regions (mean value 7.1 mM). The variation of Tmax from 270 to 890 mumol hg-1 min-1 reflected a parallel variation of total regional capillary length and surface. We conclude from the study that the cerebral capillary is a fixed unit, i.e., that the number of glucose transporters per unit of capillary surface area is the same in all regions. Regional differences are the result of different capillary densities in the regions of the brain.