Abstract: A method for preparing [18F]fluorine labeled aromatic L-amino acids is described, whereby the labeling reaction occurs at an L-enantiomeric aromatic amino acid provided with a protecting group. Furthermore, a method for preparing a diagnostic agent is described, whereby the [18F]fluorine labeled aromatic L-amino acid is used as prepared according to the invention. In addition, a method for visualizing of metabolic processes is described, whereby the [18F]fluorine labeled aromatic L-amino acid as prepared according to the invention is introduced into a living organism. Furthermore, an L-enantiomeric labeling precursor is described.