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10\g=b\-Hydroperoxy-17\g=a\-ethynyl-4-estren-17\g=b\-ol-3-one(Sch 10015) was found to be less progestational than norethynodrel but equally potent as a contraceptive agent in rats. To prevent pregnancy in all the treated females, a minimum daily dose of 5 mg./kg. body weight (MPD 100) of either agent was required. At this dose, both agents had similar contraceptive mechanisms of action. They partially blocked the release of pituitary gonadotrophin and when given on the 2nd and 3rd day after coitus, i.e. after ovulation, prevented conception. Pituitary gonadotrophin inhibition is not considered to be the primary mechanism of contraception because significant numbers of rats ovulate when treated with the MPD 100 of either compound. It is concluded that the time of the primary contraceptive action of these agents at their respective MPD 100 is post-ovulatory.It would appear from these experiments that classical progestational activity is not required for the contraceptive action of these drugs.