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Abstract— In the present study we examine the influence of pH, palmitate, and neutral amino acids on the passage of tryptophan from blood into brain during a single capillary pass, and on the partitioning of tryptophan between free and albumin-bound forms. The results show that a considerable fraction of albumin-bound tryptophan is stripped from albumin sites during passage, that uptake is concentration-dependent, and that amino acid competition for carrier sites is quantitatively the most important factor in regulating tryptophan uptake into brain. The interaction between tryptophan concentration, tryptophan binding, and competing amino acids is of considerable influence on brain serotonin biosynthesis.