The kinetics of human blood-brain barrier neutral amino acid transport sites are described using isolated human brain capillaries as an in vitro model of the human blood-brain barrier. Kinetic parameters of transport (Km, Vmax, and KD) were determined for eight large neutral amino acids. Km values ranged from 0.30 +/- 0.08 microM for phenylalanine to 8.8 +/- 4.6 microM for valine. The amino acid analogs N-methylaminoisobutyric acid and 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid were used as model substrates of the alanine- and leucine-preferring transport systems, respectively. Phenylalanine is transported solely by the L-system (which is sensitive to 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid), and leucine is transported equally by the L- and ASC-system (which is sodium-dependent and N-methylaminoisobutyric acid-independent). Dose-dependent inhibition of the high affinity transport system by p-chloromercuribenzenesulfonic acid is demonstrated for phenylalanine, similar to the known sensitivity of blood-brain barrier transport in vivo. The Km values for the human brain capillary in vitro correlate significantly (r = 0.83, p less than 0.01) with the Km values for the rat brain capillary in vivo. The results show that the affinity of human blood-brain barrier neutral amino acid transport is very high, i.e. very low Km compared to plasma amino acid concentrations. This provides a physical basis for the selective vulnerability of the human brain to derangements in amino acid availability caused by a selective hyperaminoacidemia, e.g. hyperphenylalaninemia.