Abstract: In the present study, 17β-hydroxy-7α-methylandrost-5-en-3-one was shown to terminate pregnancy following subcutaneous administration of doses of 10 mg/kg on the 1st and 2nd days or the 7th and 8th days of pregnancy in rats and on the 1st and 2nd days or 4th and 5th days in mice. The ED₅₀ of 17β-hydroxy-7α-methylandrost-5-en-3-one to terminate early pregnancy in mice was 0.159±0.055 mg/kg.Concomitant administration of megestrol acetate completely abolished the anti-pregnancy effect of 17β-hydroxy-7α-methylandrost-5-en3—one in mice, but HCG showed no such antagonism when administered in a similar way.17β-hydroxy-7α-methylandrost-5-en-3-one administered at comparable doses terminating the early pregnancy could significantly inhibit the decidual reaction in pseudopregnant mice, and this inhibition was abolished by concomitant administration of megestrol acetate.After subcutaneous injection of 17β-hydroxy-7α-methylandrost-5-en-3-one on the 7th and 8th days of pregnancy in rats, the peripheral plasma level of progesterone decreased markedly at 24hr after the administration of this drug. Results obtained from the endometrial carbonic anhydrase test of immature rabbits showed that 17β-hydroxy-7α-methylandrost-5-en-3-one was devoid of antiprogestational activity. Also, data in the viginal conification test of ovariectomized mice demonstrated 17β-hydroxy-7α-methylandrost-5-en-3-one to have an antiestrogen activity but not an estrogen activity.From the above mentioned results, it may be concluded, that: (1) 17β-hydroxy-7α-methylandrost-5-en-3-one possesses antifertility effect, (2) the effect of 17β-hydroxy-7α-methylandrost-5-en-3-one to terminate early pregnancy is related to its inhibition on deciduoma, and (3) these effects of 17β-hydroxy-7α-methylandrost-5-en-3-one are not resulted from its antiprogestational activity, but rather, may bear some relationships to reduction of the plasma level of progesterone.