Abstract: AIM　To investigate the effect of baicalin on liver microsomal cytochrome P450 system and the mechanism of liver protective action of baicalin. METHODS　Liver microsomal cytochrome P450, b5, aminopyrin N-demethylase (ADM), 7-ethoxycoumarin O-deethylase (ECD) and benzopyrene hydroxylase (AHH) activity were quantitated by UV chromatography. Activities of six cytochrome P450 isoforms were assayed with Western Blotting. RESULTS　Baicalin increased liver microsomal cytochrome P450 level and ADM, ECD and AHH activity significantly. The three P450 isoforms, 1A1, 2B1 and 2C11, were also induced selectively by baicalin, but the b5 level, 3A2, 2D1 and 2E1 were not induced. CONCLUSION　Baicalin increases liver microsomal cytochrome P450 level and induces selectively 1A1, 2B1 and 2C11 of P450 isoforms in mice.