A high-performance liquid chromatographic method for the determination of paeoniflorin, ferulic acid and baicalin in the traditional Chinese medicinal preparation Dang-Guei-San, which contains Paeoniae Radix, Swertiae Herba, Cnidii Rhizoma and Scutellariae Radix, was established. The samples ^ere separated with a Cosmosil 5C18-AR column by gradient elution with 0.03% (v/v) phosphoric acid-acetonitrile (0 min 96:4, 5 min 84:16, 7 min 82:18, 14-30 min 78:22) as the mobile phase at a flow-rate of 1.0 ml/min, with detection at 245 nm. Methylparaben was used as the internal standard and three equations were derived showing linear relationships between the peak-area ratios of marker components (paeoniflorin, ferulic acid and baicalin) to methylparaben and concentration. The recoveries of paeoniflorin, ferulic acid and baicalin were 27.86, 33.89 and 49.31%, respectively. The repeatability (relative standard deviation) was generally less than 5% (n = 5). The effects of various processes such as concentration by reduced-pressure evaporation, freeze-drying and spray-drying were studied and commercial concentrated herbal preparations containing Paeoniae Radix, Swertiae Herba, Cnidii Rhizoma and Scutellariae Radix were also analysed.