To clarify the hepatoprotective effects of tectoridin and tectorigenin from Puerariae Flos, their effects on tert-butyl hyperoxide (t-BHP)-injured HepG2 cells and mice were investigated. When tectorigenin at a dose of 50 mg/kg was intraperitoneally administered to mice injured by t-BHP, it significantly inhibited the increase the activities of plasma ALT and AST by 39% and 41%, respectively, in the t-BHP-treated group. The inhibitory effect of tectorigenin is much more potent than that of a commercially available dimethyl diphenyl bicarboxylate. Orally administered tectoridin showed hepatoprotective activity. However, when tectoridin was intraperitoneally administrated to mice, no hepatoprotective activity was observed. Tectorigenin also protected against the cytotoxicity of HepG2 cells induced by t-BHP. This protection may have originated from the inhibition of apoptosis. Tectorigenin may be hepatoprotective and tectoridin should be a prodrug that is transformed to tectorigenin.