The brain is the only organ for which amino acid transport is limited, so that competition occurs at physiological plasma concentrations. Amino acids are building stones in the synthesis of the monoamines dopamine, noradrenaline, and serotonin. Central monoamine turnover is mainly regulated by the availability of the precursor amino acids tyrosine and tryptophan in plasma. New and extended knowledge in the 21st century about the membrane bound amino acid transporters has revolutionized earlier facts about their affinity and capacity. Accordingly there are now four isoforms of system L (LAT 1-LAT 4) and three isoforms of system A (ATA1-ATA3). Each one has its own kinetic definition and can be tested by the fibroblast technique using blockers for one or more systems. Using this in vitro technique it has been found that there is a competition between the amino acids tyrosine and alanine with the major tyrosine transporter LAT 1 coded by a gene on chromosome 16. This fact has made possible a new interpretation and in all probability new understanding of earlier performed studies in patients with schizophrenia, bipolar disorder and autism.