Abstract.Scanning electron microscopy and a capillary tube sperm penetration test were used to study the cervical mucus of women using a triphasic oral contraceptive supplying a varying dosage of estrogen and gestagen during the cycle. The findings were compared with those of healthy women having regular cycles and not using contraception. Generally speaking, the ultrastructure of the cervical mucus was dense during medication, like that found in normal luteal phase. No conspicuous changes occurred with different dosages of estrogen and gestagen. For the control cycles a loose structure of the cervical mucus was found on the days prior to the temperature rise, after which a dense structure was predominant. The sperm penetration of the cervical mucus was low during medication, being less than 6 mm/h. For the controls, a high sperm penetration rate was found on the days prior to the temperature rise, while it was low in the luteal phase. The progestogenic type of cervical mucus produced by the medication at all dosages, probably contributes to the contraceptive effect.