Metabolism and pharmacokinetic studies on rat were conducted for lithospermic acid B, one of the components from Radix Salviae Miltiorrhizae (danshen) that shows many bioactivities. Liquid chromatography-electrospray ionization mass spectrometry method was applied for the determination of lithospermic acid B and its metabolites in samples from in vitro and in vivo metabolism studies. Rat plasma samples collected after intravenous administration were analyzed for obtaining pharmacokinetic data of lithospermic acid B. Four O-methylated metabolites, namely one monomethyl-, two dimethyl- and one trimethyl-lithospermic acid B, were detected when lithospermic acid B was incubated in rat hepatic cytosol. These four metabolites were also detected in rat bile, plasma and feces samples after intravenous administration of lithospermic acid B. The in vitro and in vivo results indicate that the methylation is the main metabolic pathway of lithospermic acid B. The danshen component and its methylated metabolites were excreted to rat bile and feces.