Objective: The aim of this study was to evaluate, in human volunteers, the performance of one gliclazide tablet formulation (gliclazide 80 mg tablet from EMS Industria Farmaceutica Ltda.) against two reference gliclazide tablet formulations (Diamicron 80 mg tablet from Servier do Brazil Lida. and Diamicron 80 mg tablet from Servier (Ireland) Industries Limited). Methods: The study had an open, randomized, three-period crossover design with a one-week washout interval between doses. The samples were obtained over a 48-h interval after each oral administration of gliclazide. The samples were extracted from plasma using diethylether: hexane (80: 20, v/v) and the extracts were analyzed by high-performance liquid chromatography coupled with electrospray tandem mass spectrometry (HPLC-MS/MS). Chromatography was performed isocratically using a Jones Chromatography Genesis C-8 120A 4u. The method had a chromatographic run-time of 2.5 min and a calibration curve of the range of 0.02 - 10 mu g x ml(-1) (r(2) > 0.9993). The limit of quantification was 0.02 mu g x ml(-1). Results: The geometric rnean and 90% confidence intervals (CI) for the Gliclazide/Diamicron (Ireland) ratio were 588.68% (90% CI = 491.16, 705.58%) for AUC(last), 423.50% (90% CI = 338.25, 530.23%) for AUC(inf), and 1395.77% (90% CI = 1116.62, 1744.72%) for C-max. The geometric mean and 90% confidence intervals (CI) for the Gliclazide/Diamicron (Brazil) ratio were 249.16% (90% CI = 207.96, 298.54%) for AUC(last), 249.16% (90% CI = 207.96 - 298.54%) for AUC(inf), and 188.04% (90% CI = 151.72, 233.05%) for C-max. Conclusion: Since the 90% CI for C-max, AUC(last) and AUC((0-infinity)) ratios were all outside the 80 - 125% interval proposed by the US Food and Drug Administration, we concluded that the gliclazide test formulation were not bioequivalent to either reference formulation. Interestingly, the pharmacokinetic parameters such as C-max, AUC(last) of both reference formulations are compatible with neither the literature nor the profile of an immediate release formulation. In addition, both reference formulations were not bioequivalent in themselves, indicating significant differences in reference product formulation.453175185