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Objective To develop and validate a simple,rapid,sensitive,and reproducible HPLC method for simultaneous determination of baicalein and its metabolite baicalin in dog plasma and for the subsequent pharmacokinetic study after iv administration to dogs.Methods An accurate and reproducible HPLC-UV method was developed and validated for simultaneous determination of baicalein and baicalin in dog plasma,using luteolin as internal standard.The analytes were separated by an Agilent Zorbax SB-C column(250 mm×4.6 mm,5μm)and the column temperature was maintained at 40 ℃.The mobile phase was a binary mixture of acetonitrile and water(27:73),containing 0.05% phosphoric acid in water,with a flow rate of 1.0 mL/min.The UV detector was set al 276 nm.Results Linear relationships were validated over the range of 0.05-25 μg/mL for baicalein and 0.05-20 μg/mL for baicalin.The intra-and inter-day precision values for all samples were within 8.0%,using relative standard deviation.This method was successfully applied to the pharmacokinetic studies in dogs after iv administration of baicalein.Baicalein was converted to baicalin quickly.Cmax values were 21.13 μg/mL at 0.05 h for baicalein and 1.57 μg/mL at 0.5 h for baicalin,areas under the plasma concentration-time curve were 4.97 h.μg/mL for baicalein and 0.63 h.μtg/mL for baicalin,and the elimination half-life is 0.50 h for baicalein and 0.75 h for baicalin,respectively.Conclusion The method is able and sufficient to be used in drug metabolism and pharmacoklnetic studies of baicalein.