MDMA (±3,4-methylenedioxymethamphetamine, 'ecstasy') is reportedly used recreationally because it increases feelings of sociability and interpersonal closeness. Prior work suggests that the pro-social effects of MDMA may be mediated by release of oxytocin. A direct examination of plasma levels of oxytocin after acute doses of oxytocin and MDMA, in the same individuals, would provide further evidence for the idea that MDMA produces its pro-social effects by increasing oxytocin. Fourteen healthy MDMA users participated in a 4-session, double-blind study in which they received oral MDMA (0.75 and 1.5mg/kg), intranasal oxytocin (20IU or 40IU), and placebo. Plasma oxytocin concentrations, as well as cardiovascular and subjective effects were assessed before and at several time points after drug administration. MDMA (1.5mg/kg only) increased plasma oxytocin levels to a mean peak of 83.7pg/ml at approximately 90-120min, compared to 18.6pg/ml after placebo. Intranasal oxytocin (40IU, but not 20IU) increased plasma oxytocin levels to 48.0pg/ml, 30-60min after nasal spray administration. MDMA dose-dependently increased heart rate, blood pressure, feelings of euphoria (e.g., 'High' and 'Like Drug'), and feelings of sociability, whereas oxytocin had no cardiovascular or subjective effects. The subjective and cardiovascular responses to MDMA were not related to plasma oxytocin levels, although the N was small for this analysis. Future studies examining the effects of oxytocin antagonists on responses to MDMA will help to determine the mechanism by which MDMA produces pro-social effects.