The possibility that red cell-sequestered amino acids such as phenylalanine are available for transport through the brain capillary wall, i.e., the blood-brain barrier (BBB), in vivo was investigated in the present studies with the carotid artery injection technique. Control studies included the examination of the availability of red cell-sequestered solutes such as phenylalanine or D-glucose to liver cells in vivo using a portal vein injection technique. The results show that red cell-sequestered phenylalanine is not available for transport through the BBB or into rat liver in vivo, but human red cell-sequestered D-glucose is available for uptake by liver following portal injection. Therefore, given favorable kinetics it is possible for red cell-sequestered solute to be available for uptake by tissues. However, in the case of neutral amino acids such as phenylalanine, red cell-sequestered amino acid is not available for transport through the BBB in vivo.