A new street drug, 3,4-methylenedioxy-N-methyl-butanamine (MBDB), has been found in Japan recently. The stereoisomer monitoring and the urinary excretion kinetics are not determined in biological fluids even though abused MBDB is a racemic form [enantiomer ratio (–/+) = 1.00]. The present studies were done by high-performance liquid chromatography (HPLC) equipped with a chiral activity column at 40°C using urine specimens from five Wistar rats. Urine samples were collected over six time intervals after a single oral administration of racemic MBDB (30 mg/kg). Unchanged MBDB and 3,4-methylenedioxybutanamine (BDB), an N-demethylated metabolite, were found in the rats’ urine. Each enantiomer of MBDB and BDB was monitored (peak resolution > 1.00) by HPLC analysis within 30 min. For both MBDB and BDB, the (+)-isomers were excreted a little more than the (–)-isomers. The stereoselective disposition of BDB was more remarkable than that of MBDB and was observed in the urine throughout the study (p < 0.05). The urinary excretion of MBDB showed significant difference between the two enantiomers from 4 to 20 h (p < 0.05). The amount of MBDB excreted up to 24 h was 34.7 ± 2.8% of the administered dose: 17.6 ± 1.4% for (+)-isomer and 17.1 ± 1.5% for (–)-isomer. The amount of BDB was 4.9 ± 1.0%; 2.9 ± 0.6% for (+)-isomer and 2.0 ± 0.4% for (–)-isomer. The enantiomer ratio (–/+) of MBDB and BDB was 1.00 or a little smaller. The ratio (–/+) of MBDB changed from 1.00 ± 0.02 to 0.88 ± 0.09 by 24 h, and that of BDB from 0.68 ± 0.03 to 0.78 ± 0.02. The ratio (–/+) for MBDB and BDB accumulated up to 24 h was 0.97 ± 0.01 and 0.70 ± 0.06, respectively, and the total ratio (–/+) of the two substances was 0.93 ± 0.02 (p < 0.05). These findings suggested that the stereoselective disposition of racemic MBDB was different from that of 3,4-dimethylenedioxyamphetamine and 3,4-dimethylenedioxymethamphetamine and was similar to that of methamphetamine.