The effect of cyproterone acetate on the duration of pregnancy and the number of embryos (normal, hypertrophied or degenerated) was investigated. Pregnant NMRI-strain albino mice were injected on Days 1 to 10, or Days 10 to 15 with either 0·5 or 5·0 mg cyproterone acetate (CA) or the vehicle (V). In the CA-treated animals, no parturitions occurred up to Day 21 of pregnancy. The number of offspring did, in general, not differ between the CA-treated animals and the controls. The number of normal offspring was less and the number of abnormal embryos was larger in the CA-treated animals compared with the controls. Hormonal induction of parturition showed positive results after oestrogen—oxytocin administration. The effect of CA was attributed (a) to its progestational activities, and (b) to possible direct effects on the development of the embryos.