A simple method for analysis of 5-methoxy-N,N-di-n-propyltryptamine (5-MeO-DPT) in urine has been developed using gas chromatography-mass spectrometry (GC-MS) with tetradecadeuterated 5-MeO-DPT as internal standard, which is useful for discrimination from 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT). These tryptamine designer drugs were extracted from urine with Extrelut, and derivatized with trifl uoroacetic anhydride prior to GC-MS analysis. The recovery of 5-MeO-DPT from urine was 90.7%; the calibration curve showed linearity in the range of 0.01–2.0 μg/ml. When urine samples containing two different concentrations (0.1 and 1.0 μg/ml) of 5-MeO-DPT were analyzed, the coeffi cients of variation for intraday and interday testing ranged from 3.11% to 5.82%. The established method was applied to an acute poisoning case in which 5-MeO-DIPT abuse had been suspected. Unexpectedly, 5-MeO-DIPT could not be detected, but 5-MeO-DPT was found in the subject’s urine and in an aqueous sample seized upon investigation of the poisoning; the electron-impact ionization and chemical ionization mass spectra well agreed with those of authentic 5-MeO-DPT. The concentration of 5-MeO-DPT in urine of this case was 0.37 μg/ml. Although several reports have appeared describing poisoning cases of 5-MeO-DIPT, which carries the street name “Foxy,” to our knowledge, this is the first report of GC-MS analysis of 5-MeO-DPT and demonstration of its use in a poisoning case.