In order to investigate the link between aggression and 5-HT, we looked at effects of changes in plasma tryptophan on healthy male subjects. Twenty-four with high trait aggression (H) and 24 with low (L) drank an amino acid mixture with (T+) or without (T-) tryptophan. These caused plasma tryptophan enhancement and depletion, respectively, at 4.5 h. Group H subjects given T- became more angry, aggressive, annoyed, hostile and quarrelsome on subjective measures, whereas those given T+ responded in the opposite way. On a behavioural measure of aggression, group H subjects responded more aggressively after T- than T+. In contrast, there was no consistent effect on subjective or behavioural aggression in group L subjects. Feelings of well-being in group H were decreased by T- and increased by T+. In group L, T+ reduced feelings of well-being, possibly due to the sedative effect of tryptophan in this group, which correlated positively with plasma tryptophan concentration. Changes in plasma tryptophan are probably followed by changes in central 5-HT turnover. We conclude that, in those with pre-existing aggressive traits, acute falls in central 5-HT can cause increased subjective and objective aggression, while rises can have the opposite effect. The absence of changes in a low aggressive group suggests that the primary effect may be on impulsivity, possibly mediated by 5-HT1a receptors, expressing underlying aggressive traits. The findings on mood changes provide support for earlier reports of a lowering of mood with tryptophan depletion.