The effect of oral tyrosine or tryptophan ingestion on plasma concentrations of norepinephrine (NE), dopamine (DA), epinephrine (EPI), and 3,4-dihydroxyphenylacetic acid (DOPAC) was investigated in a double blind, placebo-controlled study in fasted men. Tyrosine ingestion induced within 45 min a significant but short-lasting (approximately 30 min) increase in plasma concentrations of NE, EPI, and DA and a coincident decrease in plasma DOPAC levels. Ingestion of tryptophan or lactose placebo did not after plasma DA, EPI, NE, or DOPAC levels. Since plasma catecholamines derive from peripheral sources, while circulating DOPAC may reflect both brain and peripheral DA turnover, these results suggest that the oral ingestion of tyrosine can exert acute effects on catecholamine systems within and outside the brain.