Introduction: 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is a psychoactive indolealkylamine substance that has been used for recreational purposes and may lead to fatal toxicity. While 5-MeO-DMT is mainly inactivated via deamination, it is O-demethylated to an active metabolite, bufotenine. Quantitation of 5-MeO-DMT and bufotenine is essential in understanding the exposure to and the effects of drug and metabolite. Therefore, this study aimed to develop and validate a liquid chromatography–tandem mass spectrometry (LC–MS/MS) method for simultaneous analysis of 5-MeO-DMT and bufotenine in mouse serum. Materials & methods: A simple protein precipitation method coupled with an optimal gradient elution was used for sample preparation and separation. Detection of 5-MeO-DMT and bufotenine was accomplished using multiple reactions monitoring of m/z 219.2–174.2 and 205.2–160.2, respectively, in the positive ion mode. 5-methyl-N,N-dimethyltrypamine (m/z 203.2–158.3) was used as an internal standard for quantification. Accuracy and precision were determined after the analyses of quality control samples. Validated assay was then employed to determine drug and metabolite concentrations in serum samples collected from mice at different time points after intraperitoneal administration of 5-MeO-DMT (2 mg/kg). Results: With a total run time of 9 min, 5-MeO-DMT and bufotenine were eluted at 2.8 and 5.6 min, respectively. The assay was linear over the range 0.90–5890 ng/ml (1.12–7360 pg on-column) for 5-MeO-DMT and 2.52–5510 ng/ml (3.14–6890 pg) for bufotenine. Intra- and inter-day precision and accuracy were within 15% for both analytes. The recovery of each analyte from 20 µl of serum containing 8.08, 72.7 and 655 ng/ml of 5-MeO-DMT and 7.56, 68.1 and 613 ng/ml of bufotenine was more than 75%. Pharmacokinetic analysis revealed that the systemic exposure (area under the curve) to metabolite bufotenine was approximately 1/14 of that to 5-MeO-DMT. Conclusion: This LC–MS/MS method is a sensitive and reliable assay for quantitation of blood 5-MeO-DMT and bufotenine. Given the fact that bufotenine acts on the 5-hydroxytryptamine 2A receptor with an affinity approximately tenfold higher than 5-MeO-DMT, the active metabolite bufotenine may significantly contribute to the apparent pharmacological and toxicological effects of 5-MeO-DMT.