Two methods based on glass capillary gas chromatography mass spectrometry have been developed for quantitative determination of trioxsalen, a photosensitizing drug, in human plasma. One employs the extraction of trioxsalen and deuterated internal standard from plasma with dichloromethane and purification of the extract by high pressure liquid chromatography before analysis by selected ion monitoring at low resolution. In the other procedure the raw hexane extracts are analysed by selected ion monitoring at medium resolution. Both methods can be used to determine trioxsalen over the range 25 pg ml-1 to 2 ng ml-1, which corresponds to the range of plasma concentration present after application of trioxsalen as bath.