During a 6-month course of treatment of endometriosis with Danazol®, a synthetic derivative of 17a-ethinyl testosterone, the following hormonal changes were found. Plasma LH remained essentially unchanged but FSH conceutration was consistently in the low normal range. Plasma estradiol, especially after the fourth week of therapy, dropped to levels that did not exceed 30 pg/ml throughout the 6-month period of therapy. Plasma progesterone concentration was consistent with follicular phase levels. In the months after discontinuation of Danazol, all hormone parameters were strongly suggestive of normal ovulatory cycles. Danazol did not displace estradiol bound by endometrial cytosol.