The presence of progestogens in the cervical mucus suppresses and arrests sperm penetration. Using the Kremer technique, the effects of in vitro released progesterone, d norgestrel, and cyproterone acetate were studied by inserting silicone rubber threads bearing the respective compounds in capillary tubes containing cervical mucus. Control tubes were fitted with nonmedicated silicone rubber threads. After 24 hr of incubation, the sperm migration test was carried out to evaluate penetration depth, qualitative motility, and proportion of motile forms. Progesterone produced the greatest alterations. Migration was arrested within 30 min, the distance reached was consistently less than 2 cm, and sperm were completely immobile at 24 hr. D norgestrel also exhibited a distinct spermiostatic effect, but it was not as intense as that of progesterone. Cyproterone acetate was practically ineffective during the first 120 min, and produced alterations only in the qualitative and proportional mortility.