β Plasma phenylalanine, tyrosine, and tryptophan concentrations were measured in chronic schizophrenic patients, normal controls, and heterozygotes for phenylketonuria. Schizophrenic patients'plasma concentrations of these amino acids could not be distinguished from those of normal controls, either when fasting or following oral or intravenous (IV) phenylalanine challenge. No neuroleptic effect was observed. Plasma phenylalanine-tyrosine ratios following IV phenylalanine challenge could easily distinguish heterozygotes from schizophrenic and normal control subjects but could not distinguish schizophrenic subjects from normal control subjects. No overlap between heterozygotes' values and those of the schizophrenic and normal subjects was observed. These studies find no evidence of abnormal phenylalanine metabolism in schizophrenic persons. Phenylalanine challenge did not affect the abstraction or judgment capacities of the subjects.