A simple and sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method employing electronspray ionization was developed and validated for quantification of losartan and its carboxylic acid metabolite in human plasma using irbesartan as internal standard (IS). Following a simple pretreatment procedure, the analytes were separated using a gradient mobile phase on reverse phase C18 column. Selected reaction monitoring was specific for losartan, losartan acid and irbesartan. The method validation demonstrated the specificity, lower limit of quantification, accuracy and precision of measurements. The assay exhibited a linear dynamic range of 2.0-400 ng/mL for losartan and 1.85-370 ng/mL for losartan acid. A run time of 3.5 min for each sample made it possible to analyze more than 200 samples per day. The validated method has been successfully used to analyze human plasma samples for application in bioavailability/bioequivalence studies.