Many studies have reported correlations between measures of aggression and indices of serotonergic function, but most have studied patient or offender populations and relatively few have investigated plasma concentrations of the serotonin precursor tryptophan. This study investigates the relationship between plasma concentrations of tryptophan and trait hostility, depression and anxiety in male healthy volunteers. Sixty-seven healthy male volunteers gave blood samples and completed trait questionnaires. Plasma tryptophan was positively correlated with the Buss-Durkee Hostility Inventory Total score and Motor Aggression subscale, but not with the Attitudinal Hostility subscale or with trait anxiety or depression. In conclusion, there is evidence for an association between high concentrations of plasma tryptophan and aggressive behaviour in men, presumably mediated by some aspect of central serotonergic function, which seems unlikely to be explained by high trait anxiety or depression.