Orally active synthetic oestrogens and progestins were studied in tests to prevent pregnancy in rats. By daily administration the contraceptive effect of ethynyloestradiol and a new oestrogen, 17α-[3-furyl]-estra-1,3,5(10),7-tetraene-3,17-diol 3-acetate (AY-11483), was potentiated by three progestins, namely, chlormadinone acetate, 6-chloro-3β,17α-dihydroxypregn-4,6-dien-20-one diacetate (AY-11440) and medrogestone. AY-11483 given alone, or together with AY-11440, was more potent in preventing ovulation and implantation than ethynyloestradiol alone or in combination with other compounds. Chlormadinone acetate or AY-11440 alone were completely inactive in all the assays. The mode of action of oestrogen or oestrogen and progestin in the prevention of ovulation and implantation is discussed. In the rat, implantation seems more vulnerable to oestrogens than does ovulation.