Four biotransformation pathways of 3,4-(methylenedioxy)methamphetamine (MDMA) in the rat have been identified: N-demethylation, O-dealkylation, deamination, and conjugation (O-methylation, O-glucuronidation, and/or O-sulfation). The specific MDMA metabolites that have been identified are 3-hydroxy-4-methoxymethamphetamine, 4-hydroxy-3-methoxymethamphetamine, 3,4-dihydroxymethamphetamine, 4-hydroxy-3-methoxyamphetamine, 3,4-(methylenedioxy)amphetamine (MDA), (4-hydroxy-3-methoxyphenyl)acetone, [3,4-(methylenedioxy)phenyl]acetone, and (3,4-dihydroxyphenyl)acetone. All except 3,4-dihydroxymethamphetamine were present in the urine. The hydroxylated metabolites were excreted in the urine as the O-glucuronide and/or O-sulfate conjugates, but traces of free 4-hydroxy-3-methoxymethamphetamine and 4-hydroxy-3-methoxyamphetamine were also present in unhydrolyzed urine. N-Demethyl and 3-O-methyl phenolic amine metabolites of MDMA were consistently present in brain, liver, blood, and feces. MDMA was metabolized by the 10000g rat liver supernatant to 4-hydroxy-3-methoxymethamphetamine, 3,4-dihydroxymethamphetamine, MDA, and [3,4-(methylenedioxy)phenyl]acetone. Also, the 10000g rat brain supernatant metabolized MDMA to 4-hydroxy-3-methoxymethamphetamine, 3,4-dihydroxymethamphetamine, 4-hydroxy-3-methoxyamphetamine, and MDA.