The aim of this study was to evaluate the bioequivalence of test and reference losartan potassium 50 mg formulations in healthy volunteers. This open label, balanced, single-dose, randomized, 2-period, crossover oral bioequivalence study was conducted in 54 healthy human adult subjects under fasting condition. Subjects received losartan 50 mg of either test or reference formulation with a washout period of 7 days. After study drug administration, serial blood samples were collected over a period of 48 hours. The plasma concentrations of losartan and its active metabolite were determined by a validated method using LC/MS/MS. Pharmacokinetic parameters Cmax, Tmax, t1/2, AUC0-t, AUC 0-∞, and kel, were determined for both the  formulations. The formulations were to be considered bioequivalent if the log-transformed ratios of Cmax, AUC0-t, and AUC0-∞ were within the predetermined bioequivalence range of 80% to 125%. A total of 54 subjects were enrolled. No significant differences were found based on analysis of variance.  The mean values and 90% confidence intervals (CI) of test/reference ratios for these parameters of losartan as follows: Cmax 274.90 Vs 286.93 ng/mL (83.95 -113.69); AUC0-t 434.67 Vs 438.68 ng.hr/mL (93.30 -104.03); and AUC0-∞ 463.23 Vs 464.71 ng.hr/mL (94.65 -104.80) and for its active metabolite (losartan carboxylic acid) the mean values and 90% CI of test/reference ratios for these parameters  as follows: Cmax 572.63 Vs 543.82 ng/mL (96.87-109.37); AUC0-t 3987.89 Vs 4051.07 ng.hr/mL (93.48-104.22); and AUC0-∞ 4215.58 Vs 4271.67 ng.hr/mL (94.84-105.26). This study shows that the test formulation is bioequivalent to the reference formulation for losartan and its main active metabolite.   Key Words: Bioequivalence, Losartan, losartan carboxylic acid, Pharmacokinetics.