A method is described for the determination of a new endothelin receptor antagonist, Bosentan Ro 47-0203, in human plasma using narrow-bore liquid chromatography and ionspray tandem mass spectrometry. After protein precipitation with acetonitrile, the compounds were extracted with dichloromethane at pH 11. The compounds were chromatographed on a 2 mm I.D. reversed-phase column and introduced into the mass spectrometer with an ionspray (pneumatically assisted electrospray) interface at a flow-rate of 170 μl/min without postcolumn splitting. Two different internal standards were used for the assay: either a structural analogue or a deuterated analogue. The limit of quantification was 0.5 ng/ml using a 0.5-ml aliquot of plasma. Concentrations of the drug were determined in the range 0.5–200 ng/ml. The recovery from human plasma was 87%. The new API IIIplus collision cell was about five times more sensitive than the original API III cell. The assay was demonstrated to be sensitive, selective and robust for the analysis of over 1500 samples.