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.