A highly sensitive and selective liquid chromatography/ionspray tandem mass spectrometry (LC/MS/MS) method was developed and validated for the determination of Pranlukast and its oxidative metabolites (SB 240103, SB 241484 and SB 218663) in human plasma in order to support pharmacokinetic studies. The method employed direct injection of human plasma into an on-line solid phase extraction (SPE) PROSPEKT instrument for isolation of the analytes followed by column switching to the LC/MS/MS. The use of on-line SPE resulted in reduced sample preparation time and cleaner extracts, therefore minimizing ion suppression and HPLC back-pressures issues. The use of a 20 mM ammonium acetate-methanol system and a step gradient yielded intense ion species, excellent separation between the polar metabolites and the parent drug and sufficient selectivity for baseline resolution of the two positional isomers, SB 240103 and SB 218663. Pranlukast, its metabolites and the internal standard (SK&F 108566) were quantified using a turbo-ionspray interface by negative ion selected reaction monitoring (SRM). The lower limit of quantification (LLQ) for the assay was 10.0 ng ml-1 for Pranlukast and 1.00 ng ml-1 for its metabolites based on a 100 microliters plasma aliquot. The calibration curves were linear for analyte concentrations ranging from 10.0 to 2000 ng ml-1 for Pranlukast and 1.00 to 200 ng ml-1 for the metabolites. The calculated intra- and inter-assay precision from quality control (QC) samples resulted in mean variability values of less than 12% for all analytes. Pranlukast and its metabolites were shown to be stable under routine analysis conditions for clinical trial samples. The method provides automated sample analysis in a total cycle time of 5 min with improved robustness, sensitivity, selectivity, accuracy and reproducibility compared to the existing methodology.