This study investigated the relationship between trait hostility and aspects of serotonergic function by assessing the prolactin (PRL) response to acute tryptophan depletion and enhancement in 28 healthy male volunteers. Serum PRL was assessed immediately before, and 4.5 h after, administration of an amino acid drink enhanced with, or depleted of, the 5-HT precursor tryptophan. Trait hostility and DeltaPRL (value at 4.5 h minus baseline) correlated negatively following enhancement and positively following depletion, indicating that the higher the hostility the smaller the change in PRL in either direction. This is consistent with previous research reporting an association between aggression and blunted neuroendocrine responses to serotonergic agents. The results indicate the possibility that, in people high on hostility, part of the serotonergic pathway that leads to modulation of PRL release is characterised by a stage with either low capacity relative to input availability or a strong negative feedback component.