Abstract: A simple, highly sensitive and rapid LC-MS/MS method has been developed and validated for the quantification of metolazone in rat plasma using irbesartan as internal standard (IS). After simple protein precipitation extraction by acetonitrile, the analyte and IS were extracted from 50 μL plasma sample on an Agilent Poroshell 120, EC- C18 (50 mm × 4.6 mm, i.d., 2.7 μm) column using 5μL injection volume with a total run time of 2 min. Acidified methanol/water mixture was used as a mobile phase. The parent/product ion transitions for metolazone (m/z 366.1/258.9) and IS (m/z 429.2/207.0) were monitored on a triple quadrupole mass spectrometer, operating in the multiple reaction monitoring and positive ion mode. The method was found to be linear in the range of 0.05 – 200 metolazone. The method was validated with respect to selectivity, linearity, accuracy, precision, recovery and stability according to accepted regulatory guidelines. The described method was successfully applied to preclinical pharmacokinetic studies of analytes after an oral administration of metolazone (0.5 mg/kg) in rats.