3,4-Methylenedioxymethamphetamine (MDMA, "Ecstasy") is a recreational drug that has been shown to release serotonin (5-HT) and dopamine (DA) in animals. The effect of MDMA on 5-HT release can be blocked by 5-HT uptake inhibitors such as citalopram, suggesting that MDMA interacts with the 5-HT uptake site. It is unknown whether this mechanism is also responsible for the psychological effects of MDMA in humans. We investigated the effect of citalopram pretreatment (40 mg iv) on the psychological effects of MDMA (1.5 mg/kg po) in a double-blind placebo-controlled psychometric study in 16 healthy human volunteers. MDMA produced an emotional state with heightened mood, increased self-confidence and extroversion, moderate derealization, and an intensification of sensory perception. Most of these effects were markedly reduced by citalopram. This finding suggests that the psychological effects of MDMA are mediated via action at the 5-HT uptake site to increase 5-HT release through the carrier, as expected from animal studies.