The use of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) has increased in recent years; it can lead to life-threatening hyperthermia and serotonin syndrome. Human and rodent males appear to be more sensitive to acute toxicity than are females. MDMA is metabolized to five main metabolites by the enzymes CYP1A2, CYP2D and COMT. Little is presently known about sex-dependent differences in the pharmacokinetics of MDMA and its metabolites. We therefore analyzed MDMA disposition in male and female rats by measuring the plasma and urine concentrations of MDMA and its metabolites using a validated LC-MS method. MDA AUC(last) and C(max) were 1.6- to 1.7-fold higher in males than in females given MDMA (5 mg/kg sc), while HMMA C(max) and AUC(last) were 3.2- and 3.5-fold higher, respectively. MDMA renal clearance was 1.26-fold higher in males, and that of MDA was 2.2-fold higher. MDMA AUC(last) and t(1/2) were 50% higher in females given MDMA (1 mg/kg iv). MDA C(max) and AUC(last) were 75-82% higher in males, with a 2.8-fold higher metabolic index. Finally, the AUC(last) of MDA was 0.73-fold lower in males given 1 mg/kg iv MDA. The volumes of distribution of MDMA and MDA at steady-state were similar in the two sexes. These data strongly suggest that differences in the N-demethylation of MDMA to MDA are major influences on the MDMA and MDA pharmacokinetics in male and female rats. Hence, males are exposed to significantly more toxic MDA, which could explain previously reported sexual dysmorphism in the acute effects and toxicity of MDMA in rats.