3,4-Methylenedioxymethamphetamine (MDMA, "Ecstasy") releases serotonin and dopamine. The role for dopamine in mediating the effects of MDMA has not yet been examined in humans. We investigated the effect of pretreatment with the dopamine D(2) antagonist haloperidol (1.4 mg i.v.) on psychological and physiological responses to MDMA (1.5 mg/kg p.o.) in 14 healthy volunteers using a double-blind placebo-controlled within-subject design. Subjective peak effects were rated using standardised scales. The physiological effects measured were blood pressure, heart rate and body temperature. Side effects were assessed during the session, and after 1 and 3 days. Haloperidol attenuated MDMA-induced positive and mania-like mood but had no reducing effect on other subjective changes or on cardiovascular effects. Results are consistent with a partial dopaminergic mediation of the euphoriant effects of MDMA. In contrast, dopamine does not seem to contribute to the physiological effects of MDMA, indicating a role for serotonin and norepinephrine.