Tyrosine and tryptophan have been assayed spectrofluorometrically in postmortem human brain areas of patients with Parkinson's disease treated orally with or without 3,4-dihydroxyphenylalanine (L-dopa) plus the peripherally acting decarboxylase inhibitor benserazide. Tyrosine as well as tryptophan decrease significantly after treatment with L-dopa, thus showing a competitive action of L-dopa to other aromatic amino acids on human brain uptake. It is suggested that some of the side effects of L-dopa treatment in Parkinson's disease are due to a disturbance in the brain and neural uptake of other, specially aromatic and branched-chain amino acids. An influence of L-dopa administration on protein synthesis also cannot be excluded.