Aristolochic acid I (AA-I), which is a known nephrotoxin, is found in a commonly used Chinese medicine, Xixin, that originates from nine Asarum species (Aristolochiaceae) found in China. A method has been developed using reversed-phase liquid chromatography coupled with atmospheric pressure chemical ionization (APCI) tandem mass spectrometry under the positive ion detection mode [LC/(+)APCI/MS/MS] to determine the amount of AA-I in Xixin. The limit of detection of AA-I, estimated by monitoring with LC/MS/MS, was at the low microg/l level. By applying this method to methanol extracts of nine Asarum species, the concentrations of AA-I were found to range from 3.3 ng/mg (Asarum sieboldii) to 3376.9 ng/mg (Asarum crispulatum).