Abstract A sensitive and fast HPLC/MS/MS method for measurement of sufentanil and morphine in plasma was developed and validated. A single liquid–liquid extraction in alkaline medium was used for the cleanup of plasma, and fentanyl was added as an internal standard (IS). The analyses were carried out using a C18 column and the mobile phase acetonitrile–5 mM ammonium acetate + 0.25% formic acid (70 + 30, v/v). The triple-quadrupole mass spectrometer equipped with an electrospray source in positive mode was set up in the selective reaction monitoring mode to detect precursor → product ion transition 387.0 > 238.0, 285.7 > 165.1, and 337.0 > 188.0 for sufentanil, morphine, and IS, respectively. The method was linear in the 0.05 (LOQ) – 500 ng/mL range for sufentanil and 10 (LOQ) – 1000 ng/mL range for morphine. Good selectivity, linearity, precision, accuracy, and robustness were obtained for the HPLC/MS/MS method. The proposed method was successfully applied for the determination of sufentanil and morphine in patients undergoing cardiac surgery.