The pharmacokinetics and metabolism of lithospermic acid (LA), a component isolated from Salvia miltiorrhiza, and its two O-methylated metabolites (3'-monomethyl- and 3',3''-dimethyl-lithospermic acid), were analyzed by a rapid and specific isocratic liquid chromatography-tandem mass spectrometry (LC/MS/MS) method. Rat serum samples collected after intravenous and oral administration were analyzed for obtaining pharmacokinetic data of LA. Two O-methylated metabolites, namely one 3'-monomethyl- and one 3',3''-dimethyl-lithospermic acid were detected in rat serum and bile samples after intravenous and oral administration of LA, respectively. An oral bioavailability of 1.15% was found, with the AUC(0-t) values of 301.89 and 3.46mgh/L for intravenous and oral administration, respectively. The total recovery from bile was 75.36% (0.46% for LA, 17.23% for M1, and 57.67% for M2) after intravenous administration, and 4.26% (0.00% for LA, 0.10% for M1, and 4.16% for M2) after oral administration. These results indicate that methylation is the main metabolic pathway of LA, and that LA is excreted into rat bile and finally into feces.