The crude alkaloidal fraction of the root of Cynanchum caudatum Max.(Asclepiadaceae) was tested for the effects on the activities of free radical generating enzymes and the formation of lipid peroxide. Aldehyde oxidase was strongly inhibited to about 90% of the activity by treating 1.0 mg/ml of alkaloidal fraction, corresponding to competitive inhibition. Moreover, the formation of lipid peroxide which causes damage of cell membrane was reduced in proportion to the increasing alkaloid concentration. However, xanthine oxidase of which structure and function are similar to those of aldehyde oxidase was not inhibited by the alkaloidal fraction.