Evaluation of sex steroids in cervical mucus was performed at different phases of spontaneous or clomiphene-citrate-induced ovulatory cycles. To this end, 11 women with normal ovulatory cycles and 9 subjects with polycystic ovary syndrome of comparable age and body mass index were investigated. Serum and cervical mucus samplings were assessed for 17beta-estradiol (E2), progesterone, testosterone, and sex hormone binding globulin levels at the pre-, peri-ovulatory, and mid-luteal phases of the cycle. The cervical mucus maturation index also was estimated in all women. Measurable amounts of E2 were found in most mucus samples with a cyclic variation in all cases. The highest E2 and mucus maturation index values coincided, but both lagged by 24 h behind the serum mid-cycle peak of this steroid. Detectable amounts of progesterone were found in the luteal phase, testosterone was present at low levels throughout the cycle, but sex hormone binding globulin was undetectable in all cervical mucus samples. Differences between spontaneous or drug-induced ovulatory cycles were not found. It is concluded that sex steroids are present in human cervical mucus, showing variations similar to those in peripheral blood. The significance of these findings is not clear at present, but it is probably related to the cyclic changes of cervical epithelium and gland secretion. An important implication of the absence of measurable sex hormone binding globulin amounts in cervical mucus is that the free fraction of sex steroids present in that fluid are presumably higher, and therefore, expected to exert greater biologic activity than in peripheral blood.