A liquid chromatography-electrospray ionization tandem mass spectrometric method was developed for the simultaneous determination of losartan and its major active metabolite, EXP-3174, in human plasma. The two analytes and the internal standard (DuP-167) were extracted from plasma under acidic conditions by using solid-phase extraction cartridges containing a sorbent of copolymer, poly(divinylbenzene-co-N-vinylpyrrolidone). The analytes were separated by LC equipped with a reversed-phase C18 column, and introduced into the mass spectrometer via the electrospray ion source with pneumatically-assisted nebulization. For LC-MS-MS samples, an isocratic mobile phase consisting of [0.1% triethylamine-0.1% acetic acid (pH 7.1)]-acetonitorile (65:35, v/v) was used, and the assay was monitored for the negative fragment ions of the analytes. The method demonstrated linearity from 1 to 1000 ng/ml for both losartan and EXP-3174. The limit of quantification for both compounds in plasma was 1 ng/ml. This assay method may be useful for the measurement of levels of the two compounds in clinical studies of losartan.