Abstract: A specific and sensitive method has been established for the analysis of pseudoephedrine (PE) in human plasma by gas chromatography with a mass selective detector (GCMSD). PE and ephedrine (E, internal standard) reacted with trifluoroaeetie anhydride (TFAA) and the derivatives were separated by a HP-5 capillary column. PE-TFA and E-TFA were quantitatively measured by selected ion monitoring (m/z=154). The detection limits for Eand PE were 1 ng/ml plasma. The recoveries were about 99%.