A validated method based on liquid chromatography/positive ion electrospray-mass spectrometry (LC-ESI/MS) is described for the quantification of perindopril and its active metabolite, perindoprilat, in human plasma. The assay was based on 500-microL plasma samples, following solid-phase extraction using Oasis HLB cartridges. All analytes and the internal standard (trandolapril) were separated by hydrophilic interaction liquid chromatography using a SeQuant Zic-HILIC analytical column (150.0 x 2.1 mm i.d., particle size 3.5 microm, 200 A) with isocratic elution. The mobile phase consisted of 10% 5.0 mM ammonium acetate water solution in a binary mixture of acetonitrile/methanol (60:40, v/v) and pumped at a flow rate of 0.10 mL min(-1). Quantitation of the analytes was performed with selected ion monitoring (SIM) in positive ionization mode using electrospray ionization interface. The assay was found to be linear in the concentration range of 5.0-500.0 ng mL(-1) for perindopril and perindoprilat. Intermediate precision were found less than 3.5% over the tested concentration ranges. A run time of less than 6.0 min for each sample made it possible to analyze a large number of human plasma samples per day. The method is the first reported application of HILIC in the analysis of angiotensin-converting enzyme inhibitors and can be used to quantify perindopril and perindoprilat in human plasma covering a variety of pharmacokinetic or bioequivalence studies.