Various contraceptive steroids were analyzed for their oestrogenic potency in the human using changes in the cortisol-binding capacity of transcortin (TC-BC) as a parameter. A linear dose-related elevation of TC-BC was recorded in response to increasing amounts of ethinyloestradiol (EE). Mestranol (EEME), given in the same galenical preparation as EE, proved to be only 64 % as potent. Megestrol acetate, dl-norgestrel, norethindrone and its acetate failed to influence TC-BC, while norethynodrel induced a significant rise. This effect is apparently due to both the inherent oestrogenic activity of norethynodrel and its contamination with EEME. The administration of combined oral contraceptives resulted in a significant elevation of TC-BC. The drugs studied were (in order to increasing total oestrogenicity) 50 μg EEME + 1 mg norethindrone, 35 μg EE + 0.25 mg d-norgestrel, 100 μg EEME + 2 mg norethindrone, 50 μg EE + 0.25 mg d-norgestrel, and 100 μg EEME + 2.5 mg norethynodrel. The observed rise in transcortin activity appears to depend primarily on the chemical structure and dosage of the oestrogen component. These findings clearly indicate that the administration of conventional oral contraceptives, even those containing low oestrogen doses, does not cause an oestrogen deficiency, but rather an oestrogen oversupply of the female organism.