dA controlled administration study was conducted on the closed research unit of the National Institute on Drug Abuse Intramural Research Program to examine the pharmacodynamics (n=8) and pharmacokinetics (n=17) of the popular recreational drug 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) in experienced ecstasy users, ages 18-27 years, for ≤ 168 h after dosing. Placebo, 1.0 (low) and 1.6 (high) mg/kg MDMA were administered in a randomized, within-subject, double blind manner. An analytically sensitive and specific 2D-GC/EI-MS with cryofocusing was developed and validated for simultaneous quantification of MDMA and metabolites, 3,4-methylenedioxyamphetamine (MDA), 4-hydroxy-3-methoxymethamphetamine(HMMA) and 4-hydroxy-3-methoxyamphetamine (HMA) in human plasma. Calibration curves were 1-100 ng/mL MDA, 2.5-100 ng/mL HMA and 2.5-400 ng/mL MDMA and HMMA. Accuracy was within +/- 20% of target, and intra- and inter-assay imprecision were < 8.5%. Peak effects were recorded 1-2 h post-dose. Cardiovascular parameters significantly increased. Temperature increased (non-significantly) by ≤ 1.9°C. Mydriasis occurred after active MDMA doses. Significant increases in visual analog scales characterizing the stimulatory, hallucinogenic and entactogenic effects of MDMA were observed. Although most pharmacodynamic measures significantly correlated to MDMA plasma concentrations, all correlation coefficients were ≤ 0.5, making prediction of effects from a single plasma concentration unreliable. Non-linear pharmacokinetics was observed. Cmax of all analytes except HMMA were significantly higher after the high dose. Times of last plasma detection were generally HMA<MDA<MDMA<HMMA. Mean half-lives (t 1/2) were approximately: MDMA, 7-8 h; HMMA, 11.5-13.5 h; MDA 10.5-12.5 h. HMA t1/2 showed high variability. Mean MDMA volume of distribution was constant between doses; clearance was significantly higher after the low dose. Gender differences in some pharmacokinetic parameters were observed for select analytes and doses.;Stimulatory, hallucinogenic and entactogenic effects were documented after 1.0 and 1.6 mg/kg MDMA administration. The largest pharmacokinetic examination of MDMA to-date was performed, and included for the first time, data from African-Americans and females, measurement of HMMA and HMA concentrations after low and high MDMA doses and plasma collections beyond 48 h. Characterizing the pharmacodynamics and pharmacokinetics of MDMA provides information on possible mechanisms involved in its toxicity and reasons for abuse and may contribute to drug abuse prevention and treatment efforts.