Abstract: t Aim ：To study the distribution and excretion of baicalein and baicalin in rats. Methods ：An LC/MS method was applied.Chromatographic separation was performed on a C₁₈ column(150 mm×6.0 mm，5 μm) with methanol-10 mmol/L ammonium acetate (containing 0.5% formic acid) as the mobile phase.A trip-quadrupole tandem mass spectrometer was set in positive selected reaction monitoring mode.The sample was extracted with methanol-acetonitrile(1∶1) after the addition of phosphoric buffer solution and luteolin，which acted as the internal standard.The supernatant was evaporated to dryness，and the residual was reconstituted with mobile phase and analyzed. Results ：The distribution profiles of the parent drug and its main metabolite showed two peaks between 20-40 min and 8-10 h after oral administration of baicalein，which fit the plasma concentration-time profile of baicalein in rats.At 20 min after the dosing，the concentration levels of baicalein were significantly higher than those of baicalin in stomach，liver and intestines，the converse result occurred in kidney.The excretion results showed that baicalin was the predominant excretion form in bile and urine，while baicalein was the negligible excretion form.There was more baicalein than baicalin in rat feces. Conclusion ：Baicalein was absorbed and distributed quickly to various tissues and easily transformed to its metabolite at the same time.