Abstract: Quantitative analysis of(l)-ephedrine(l-Ep)and(d)-pseudoephedrine(d-Ps )in plasma by high-performance liquid chromatography(HPLC)is described.The newly developed method is based on a precolumn derivatization with 5-dimethyl- aminonaphthalene-1-sulfonyl chloride(DNSCI )in acetonitrile. The diastereomers formed were separated on a reversed phase column by HPLC with fluorescence detection employing 0.6%phosphate buffer(pH 6.5)-methanof(3:8,v/v)as mobile phase.The detection limit of each Ephedra alkaloid stereoisomer was 0.5 ng at a signal- to-noise ratio of 3 :1 ,the linear response to each stereoisomer being 1～800 ng· ml^-1.The plasma level profile of l-Ep and d-ps in guinea pig was investigated by this method. The determination of l- Ep and d-Ps in plasma of a volunteer after oral administration of Xiao Qinglong Heji was also performed.