The effect of a carbohydrate or a 20% protein (HP) or a carbohydrate + 0.4% tryptophan (TRP) evening meal on plasma amino acids and on plasma neutral amino acid (NAA) ratios was studied in eight healthy men. After consumption of the protein meal, plasma amino acids rose after 1 h and remained at the same level at 2 h. The dietary profile of the essential amino acids, except for TRP, was retained in the plasma. The plasma profile of nonessential amino acids was not related to the dietary pattern. Glutamic acid and aspartic acid increased several-fold less and alanine several-fold more than would be expected from their dietary concentrations. The tyrosine/NAA and phenylalanine/NAA ratios were not altered by any of the meals and the TRP/NAA ratio was increased only by the carbohydrate + 0.4% TRP meal. The leucine/NAA and isoleucine/NAA ratios were decreased by the carbohydrate and carbohydrate + TRP and increased by the 20% protein meals, while valine/NAA was decreased by the carbohydrate + TRP and HP but increased by the carbohydrate meal. From these data it is concluded that if brain neurotransmitter synthesis is controlled by the same mechanisms as in rats, then the evening meal containing added TRP was the only one of the meals likely to affect brain neurotransmitter (serotonin) synthesis.