3,4-Methylenedioxymethamphetamine (MDMA; "ecstasy" ) is a selective serotonin (5-HT) neurotoxin in laboratory animals. To assess its effects on 5-HT function in humans, serum prolactin (PRL) and mood responses to intravenous L-tryptophan were measured in nine recreational users of MDMA and compared with findings from nine matched healthy controls. L-Tryptophan induced a rise in the PRL concentration in controls, but not in MDMA users. Peak change and the area under the curve of the PRL response appeared to be blunted in MDMA users, but the difference from controls did not reach statistical significance. This study provides suggestive evidence of altered 5-HT function in MDMA users, but more definitive studies clearly are needed.