A rapid and sensitive method based on liquid chromatography/tandem mass spectrometry (LC/MS/MS) was developed for the simultaneous determination of lithospermic acid B and its three main O-methylated metabolites in rat serum with silibinin as the internal standard. The calibration curves for LSB, and the three metabolites were linear over the ranges of 16-4096, and 8-2048 ng/ml, respectively, with coefficients of correlation >0.998. For LSB, the intra-assay coefficient of variance (CV) was less than 9.3% and the inter-assay CV was less than 8.9%. The inter-assay mean accuracy was between 92.8% and 104.7%. For the three metabolites, the intra-assay CV was less than 8.7% and the inter-assay CV was less than 9.9%. The inter-assay mean accuracy was between 92.5% and 107.9%. This quantitation method was successfully applied to a pharmacokinetic study of LSB in rats. Also, a total recovery of 5.2% was found in bile after oral administration.