The dipeptide aspartame (APM; aspartylphenylalanine methylester), an artificial sweetener, was studied in vivo for its ability to influence brain levels of the large neutral amino acids and the rates of hydroxylation of the aromatic amino acids. The administration by gavage of APM (200 mg/kg) caused large increments in blood and brain levels of phenylalanine and tyrosine by 60 minutes. Brain tryptophan level was occasionally reduced significantly, but the brain levels of the branched-chain amino acids were always unaffected. Smaller doses (50, 100 mg/kg) also raised blood and brain tyrosine and phenylalanine, but did not reduce brain tryptophan levels. At the highest dose (200 mg/kg), APM gavage caused an insignificant increase in dopa accumulation (after NSD-1015), and a modest reduction in 5-hydroxytryptophan accumulation. No changes in the brain levels of serotonin, 5-hydroxyindoleacetic acid, dopamine, dihydroxyphenylacetic acid, homovanillic acid, or norepinephrine were produced by APM administration (200 mg/kg). These results thus indicate that APM, even when administered in amounts that cause large increments in brain tyrosine and phenylalanine, produce minimal effects on the rates of formation of monoamine transmitters.