Abstract: The alcoholic extract of Aristolochia mollissima H., a Chinese medicinal herb, was reported to have anti-implantation activity in mice and rats. Aristolochic acid A, a nitrophenanthrene, was obtained from the plant by extracting with acidic chloroform, followed by chromatography on silica gel columns. Aristolochic acid A given orally to mice showed significant anti-implantation and early pregnancy interrupting effects. However, these effects were not observed in rats. Aristolochic acid A showed neither estrogenic nor antiestrogenic actions. Treatment with exogenous progesterone failed to provent its pregnancy-interrupting action. In addtion, intraamniotie injection of Aristolochie acid A in mid-term pregnant dogs and rats led to termination of pregnancy. No significant changes were found in blood counts, blood sugar, cholesterol, SGOT, alkaline phosphatase, acid phosphatase, fibrinogen content, thrombinogen time, hepatic and renal functions as well as the morphology of the main viscera when Aristoloehie acid A was administered intraamniotically to dogs at a single dose in the range of 0.2～2 mg.