A method is described for the simultaneous determination of the ratio of l- and d-enantiomers of amphetamine, methamphetamine, 3,4-methylenedioxyamphetamine (MDA), 3,4-methylenedioxymethamphetamine (MDMA), and 3,4-methylenedioxyethylamphetamine (MDEA) in urine. The assay uses liquid-liquid extraction followed by derivatization with trifluoroacetyl-l-prolyl chloride (l-TPC) and analysis by gas chromatography-mass spectrometry. The assay was developed using prepared samples containing varying concentrations of each of the analytes over a range of percentages of each enantiomer. Results showed the method to provide accurate and reliable results in samples containing > or = 10 ng/mL amphetamine and methamphetamine and > or = 25 ng/mL MDA, MDMA, and MDEA. The assay was used to analyze urine samples from subjects of a controlled MDMA study. Results for each of the eight subjects showed a greater percentage of the l-enantiomer of MDMA initially, and the percentage increased with time postdose. Analysis of the metabolite MDA revealed that the proportion of d-enantiomer was initially greater than the l-enantiomer followed by a gradual increase in the proportion of l-enantiomer until it exceeded the amount of the d-enantiomer. In all cases, the l-MDA exceeded the d-MDA within the first 36 h postdose.