This study compares the interactive emotional/subjective effects of single doses of 3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") and methylphenidate, a dopamine (DA) and norepinephrine (NE) transporter blocker, in healthy subjects. The primary goal is to determine the role of transporter mediated DA and NE release in the subjective response to MDMA in humans. The investigators hypothesize that methylphenidate will attenuate the subjective response to MDMA.