Cathinone-derived designer drugs have recently grown to be popular as drugs of abuse. 3,4-Dimethylmethcathinone (DMMC) has recently been abused as one of the alternatives to controlled cathinones. In the present study, DMMC and its major metabolites, 3,4-dimethylcathinone (DMC), 1-(3,4-dimethylphenyl)-2-methylaminopropan-1-ol (beta-OH-DMMC, diastereomers), and 2-amino-1-(3,4-dimethylphenyl)propan-1-ol (beta-OH-DMC, diastereomers), have been identified and quantified in a DMMC user's urine by gas chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry using newly synthesized authentic standards. Other putative metabolites including oxidative metabolites of the xylyl group and conjugated metabolites have also been detected in urine. The identified and putative phase I metabolites indicated that the metabolic pathways of DMMC include its reduction of the ketone group to the corresponding alcohols, N-demethylation to the primary amine, oxidation of the xylyl group to the corresponding alcohol and carboxylate forms, and combination of these steps. Concentrations of the identified metabolites were found to increase slightly after enzymatic hydrolysis, suggesting that these compounds are partially metabolized to the respective conjugates.