The purpose of this study is to measure the effects of MDMA (particularly its emotional effects) and to determine the role of serotonin in these effects. Serotonin is a neurotransmitter, which is a chemical that is released by some brain cells to communicate with other brain cells. Many of the effects of MDMA are thought to be the result of increased serotonin release.

In order to understand the effects of MDMA and role of serotonin in these effects, we will administer MDMA alone and in combination with the antidepressant citalopram (one trade name for this is Celexa). Citalopram decreases the ability of MDMA to release serotonin. Citalopram will therefore decrease any of MDMA's effects that are the result of serotonin release; we want to measure this.

Inclusion Criteria:

• Healthy men or women aged 18 to 50 years
• Experienced with MDMA

Exclusion Criteria:

• Significant physical or psychiatric illness which might impair the ability to safely complete the study or that might be complicated by the study drugs, including prior seizures (after age 8), history of major depression, or other active neurological disease or clinically significant abnormalities on physical examination or screening laboratory values