Measurements of the unidirectional blood-brain glucose flux in rat were incompatible with a single set of kinetic constants for transendothelial transport. At least two transfer mechanisms were present: a high-affinity, low-capacity system, and a low-affinity, high-capacity system. The low-affinity system did not represent passive diffusion because it distinguished between D- and L-glucose. The Tmax and Km for the high-affinity system were 0.16 mmol 100 g-1 min-1 and 1 mM; for the low-affinity system, approximately 5 mmol 100 g-1 min-1 and approximately 1 M. With these values, physiological glucose concentrations were not sufficient to saturate the low-affinity system. In normoglycemia, therefore, three independent pathways of glucose transport from blood to brain appear to exist: a high-affinity facilitated diffusion pathway of apparent permeability 235 X 10(-7) cm s-1, a specific but nonsaturable diffusion pathway of permeability 85 x 10(-7) cm s-1, and a nonspecific passive diffusion pathway of permeability 2 x 10(-7) cm s-1.