Hepatic lipid peroxide contents were examined in bromobenzene-treated rats firstly after the oral administration of MeOH extract ofKalopanax pictus stem bark, its n-BuOH fraction, EtOAc fraction and an alkaline hydrolysate of the n-BuOH fraction, and secondly after the intraperitoneal administration of hederagenin monodesmosides and bisdesmosides. Two hederagenin monodesmosides, kalopanaxsaponin A (KPS-A) and sapindoside C, exhibited significant anti-lipid peroxidation effects after intraperitoneal administration at doses of 10-30 μmole/kg, whereas their bisdesmosides did not exhibit any significant activity. These results suggest that it is the hederagenin monodesmosides that are responsible for anti-lipid peroxidationin vivo. The activity of KPS-A was established by the observation of decreased aminopyrine N-demethylase activity and increased epoxide hydrolase activity.