Buyang Huanwu decoction (BYHWD) is a widely used Chinese traditional compound medicine that has proved effective in treating cerebrovascular illnesses; however, its active substances have remained unknown. In this paper, serum chemistry and combined high-performance liquid chromatography (HPLC), photodiode-array detection and mass-spectrometry techniques were used to study the constituents of BYHWD from pig serum after oral administration. A total of 45 characteristic HPLC peaks were detected from serum containing drug. The chemical structures of nine of the peaks were tentatively elucidated as 7,3'-dihydroxy-4'-methoxyisoflavone-7-O-glucuronide (P1), 7-hydroxy-4'-methoxyisoflavone-7-O-glucuronide (P2), 7,2',4'-trihydroxy-3'-methoxyisoflavane-7-O-sulphate (P3), 3-hydroxy-9,10-dimethoxypterocarpan-3-O-glucuronide (P4), 7,2'-dihydroxy-3',4'-dimethoxyisoflavane-7-O-glucuronide (P5), 3-hydroxy-9,10-dimethoxypterocarpane-3-O-sulphate (P6), 4(1H)-quinolinone (P7 or P8), 4-hydroxyquinoline (P8 or P7) and oleic acid (P9). All of the identified peaks, with the exception of P9, were metabolites of the constituents of BYHWD in vivo.