Gagaminine, a steroidal alkaloid, was isolated for the first time from the root of Cynanchum wilfordi Hemsley (Asclepiadaceae), and its effects on lipid peroxidation and the activity of aldehyde oxidase (EC. 1.2.3.1) were investigated in vitro. This alkaloid suppressed significantly the formation of lipid peroxides in rat liver tissues and potently inhibited the hepatic aldehyde oxidase activity in a dose-dependent manner, with IC50 value of 0.8 microM (0.5 microg/ml). These results indicate that gagaminine is a potent natural antioxidant and may be useful for clinical tests.