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A simple and rapid analytical method was developed and validated for the simultaneous determination of pirfenidone and its metabolite 5–carboxy–pirfenidone in human plasma using liquid chromatography–tandem mass spectrometry.Pirfenidone and 5–carboxy–pirfenidone as well as the deuterium–labeled internal standards (IS) were deproteinizated with acetonitrile using an aliquot of 0.1 ml plasma sample.LC conditions utilized an Agilent ZOBAX Plus C18 column with an isocratic elution of acetonitrile and 0.1 % formic acid in 5 mM ammonium formate aqueous solution (60:40,v/v).The transition of m/z 186.1→65.1,m/z 191.1→65.1,m/z 216.0→77.0,and m/z 221.0→81.0 under multiple reaction monitoring in positive ionization mode was chosen to quantify pirfenidone,5–carboxy–pirfenidone and IS,respectively.The analysis time was within 3 min.The calibration curve was linear over the concentration range of 0.005?25 μg/mL for pirfenidone and 0.005?15 μg/mL for 5–carboxy–pirfenidone.The lower limit of quantification (LLOQ) was both 0.005 μg/mL.The intra– and inter–day precision and relative errors of quality control (QC) samples were from–11.7 % to 1.3 % for pirfenidone and from -5.6 % to 2.5 % for 5-carboxy-pirfenidone with mean recoveries up to more than 90%.The method was validated as simple,sensitive,and rapid and has been successfully applied to investigate the pharmacokinetics of pirfenidone tablets in healthy humans.