The effect of lowering or raising tryptophan levels was studied in normal males using amino acid mixtures that were tryptophan free or tryptophan supplemented. Tryptophan depletion caused a small but significant alteration in food selection in subjects allowed to select from a buffet. Although carbohydrate and total kilocalories selected were unchanged, significantly less protein was chosen. Tryptophan depletion also caused an acute lowering of mood, suggesting that low serotonin (5-HT) may be involved in the etiology of clinical depression in some patients. No effect of altered tryptophan levels was seen in a laboratory test of aggression. However, a study on vervet monkeys indicated that altered tryptophan levels can influence aggression when the animals are at a high level of arousal. High arousal is known to increase the firing rate of 5-HT neurons. In a preliminary study, tryptophan had a therapeutic effect in aggressive schizophrenic patients. The best effect was seen in impulsive patients, which may have been related to high arousal in these subjects. Although carbohydrate is capable of raising brain tryptophan, not all carbohydrate-induced behaviors are mediated by tryptophan. Thus, sucrose was capable of attenuating alcohol intoxication in normal human males without altering blood alcohol concentrations. However, tryptophan had no effect on ethanol intoxication. The challenge for the future is to define the conditions under which alterations in tryptophan levels can influence brain function.