Carbohydrate ingestion raises tryptophan uptake and serotonin synthesis in rat brain. The addition of protein is generally believed only to block such increases. However, some recent evidence suggests dietary protein may not be limited to this action. In the present studies, we fed rats single meals containing one of 5 proteins (zein, wheat gluten, soy protein isolate, casein, lactalbumin, 17% by weight) or no protein, and killed them 2.5 h later, 30 min after the injection of m-hydroxybenzylhydrazine, to allow serotonin and catecholamine synthesis rates to be measured in brain. Blood and cerebral cortex samples were analyzed for tryptophan and other large, neutral amino acids; 5-hydroxytryptophan and dihydroxyphenylalanine were measured in hypothalamus, hippocampus and cerebral cortex as indices of serotonin and catecholamine synthesis, respectively. An 8-fold variation occurred in cortex tryptophan: a marked decline followed zein ingestion, and modest reductions after casein or gluten. A large rise in cortex tryptophan occurred after lactalbumin consumption, and smaller increases after soy protein or carbohydrate (no protein). In the brain regions examined, a 4-8-fold range in serotonin synthesis occurred which closely followed the tryptophan alterations. No effects were observed in regional catecholamine synthesis rates. Cortical concentrations of leucine showed small changes; leucine has been linked to mTOR (mammalian target of rapamycin) signaling in brain circuits regulating food intake. The data suggest that tryptophan concentrations and serotonin synthesis in brain neurons are remarkably sensitive to which protein is present in a meal. Conceivably, this relationship might inform the brain about the nutritional quality of the protein ingested.