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Objective: To develop a high performance liquid chromatography mass spectrometry (HPLC-MS) method for the determination of the glycyrrhetic acid (GA) in human plasma and for the investigation of its pharmacokinetics after the oral administration of 150 mg diammonium glycyrrhizinate test and reference capsule formulations., Methods: The GA in plasma was extracted with ethyl acetate, separated on a C(18) column with a mobile phase of methanol (5 mmol/L ammonium acetate)-water (85 : 15, V/V) and analysed using a MS detector. Ursolic acid (UA) was used as internal standard. The target ions were m/z 469.5 for GA and m/z 455.6 for UA, the fragment voltages were 200 V and 100 V for GA and UA respectively., Results: The calibration curve was linear over the range of 0.5-200 ng/mL (r = 0.9974). The limit of quantification for GA in plasma was 0.5 ng/mL, the recovery was 76.0-80.0%, and the inter- and intra-day relative standard deviations (RSD) were <12%. The pharmacokinetic parameters of GA after a single dose of 150 mg diammonium glycyrrhizinate test and reference were as follows: the half life (t(1/2)) 9.65 +/- 3.54 h and 9.46 +/- 2.85 h, the time to peak concentration (T(max)) 10.95 +/- 1.32 h and 11.00 +/- 1.30 h, the peak concentration (C(max)) 95.57 +/- 43.06 ng/mL and 103.89 +/- 49.24 ng/mL; the area under time-concentration curve (AUC(0-48) and AUC(0-infinity)) 1281.84 +/- 527.11 ng.h/mL and 1367.74 +/- 563.27 ng.h/mL, 1314.32 +/- 566.40 ng.h/mL and 1396.97 +/- 630.06 ng.h/mL. The relative bioavailability of diammonium glycyrrhizinate capsule was 98.88 +/- 12.98%., Conclusion: The assay was sensitive, accurate and convenient, and can be used for the determination of GA in human plasma. Comparison of the bioavailability and pharmacokinetic profile of GA indicated that the test and reference capsules were bioequivalent.