The roots of Cynanhum caudatum (Asclepiadaceae) have been used in folk medicine in Japan and China for the prevention and treatment of various geriatric diseases and also as a cardiotonic agent. Constituents of this plant have mainly been examined for glycosides: besides two steroidal alkaloids, gagaminine and gagamine which was firstly isolated by us, more than 35 polyoxypregnane glycosides and aglycones have also been identified. Gagaminine inhibits potently the hepatic aldehyde oxidase activity and lipid peroxidation in vitro. The present work deals with the comparison of antioxidative activities of gagamine, a new pregnane alkaloid, three isolated polyoxypregnanes containing a keto group at C-20 with those of gagaminine, a potent antioxidant, in order to explain the structure-activity relationships. The results of this study further prove that the cinnamoyl group of gagaminine is very important for the inhibition on the aldehyde oxidase activity while the nicotinoyl group is necessary for anti-lipid peroxidation. Besides that, the keto compounds having no ester group at C-12 were found to be more active than the others except gagaminine.