An analytical procedure for the determination of the new pain medication tapentadol and its main metabolite N-desmethyltapentadol (DMT), in urine and oral fluid has been developed and validated using liquid chromatography with tandem mass spectral detection (LC-MS-MS). Oral fluid was collected using Quantisal™ devices, and drugs present were quantified using solid-phase extraction followed by LC-MS-MS. For confirmation, two transitions were monitored and one ratio determined which had to be within 20% of that of the known calibration standard. For tapentadol, 222.1 > 107 was used as the quantifying transition; 222.1 > 121 for the qualifier. For DMT, 208.1 > 107 was used for quantification; 208.1 > 121 as the qualifier. For saliva, the linear range was 10-100 ng/mL; the lower limit of quantitation (LLOQ) was 10 ng/mL; the intraday precision was 3.6% (n = 6) and interday precision was 13.6% (n = 24). The recovery of tapentadol and DMT from the oral fluid collection pad was > 99%. For urine, the specimens were diluted and injected directly into the LC-MS-MS. The LLOQ was 50 ng/mL; the intraday and interday precisions were 2.1% and 4.4%, respectively, for tapentadol and 2.9% and 5.7%, respectively, for DMT. This is the first analytical procedure for tapentadol and DMT in urine and oral fluid.