Abstract: In the present study, 7α-methyl-10β, 17β-diacetoxy-A4-estren-3-one were shown to be able to terminate pregnancy following subcutaneous administration on the 7 th and 8 th days of pregnancy in rats, and on the 4 th and 5 th days in mice. The ED₅₀ of 7a-methyl-10β, 17β-diacetoxy-△⁴-estren-3-one for the interruption of early pregnancy in mice was 1.6 mg/kg. Under similar experimental condition the ED₅₀ of 7β-methyl-10β, 17β-diacetoxy-△⁴-estren-3-one was found to be 5.5 mg/kg. After administration of effective dose of 7α-methyl-10β, 17β-diacetoxy-△⁴-estren3-one to early pregnant rats, the peripheral plasma progesterone level was decreased markedly. At the concentration of 10 μg/ml, 7α-methyl-10β, 17β-diacetoxy-△⁴-stren-3-one showed significant inhibiting action on progesterone biosynthesis in isolated pregnant rat ovary. Relative binding affinity (RBA) of 7a-methyl-lOft, 17β-diacetoxy-△⁴-estren-3-one for rabbit uterine estrogen receptor was found to be 10.8 and that of 7β-methyl-10β, 17β-diacetoxy-△⁴-estren-3-one to be 1.5. However, 7△-and 7β-methyl-10β, 17β-diacetoxy-△⁴-estren-3-one were of poor affinity for progesterone binding sites. Result obtained from the uterine growth test in immature mice showed that the estrogen and antiestrogen activity of 7β-methyl-10β, 17β-diacetoxy-△⁴-estren-3-one was weaker than that of 7α-methyl-10β, 17β-diacetoxy△⁴-estren-3-one.