The serum concentrations of ethinylestradiol (EE) and 3-keto-desogestrel (KDG) were compared during treatment with a combination of 20 µg EE + 150 µg DG (20EE/DG) or of 30 µg EE + 150 µg DG (30EE/DG). During intake of both preparations, the peak levels and the areas under the curve (AUC) of EE increased significantly by approximately 100% between days 1 and 10. In the steady state, the maximal EE levels were 75 ± 34 pg/ml (20EE/DG) and 136 ± 55 pg/ml (30EE/DG), and the AUC were 464 ± 236 pg·h/ml and 840 ± 492 pg·h/ml. The KDG levels, which were identical with both preparations, increased between days 1 and 21 by approximately 300% up to values of 4.5 ± 1.6 ng/ml. There were large interindividual variations in the AUC of EE and KDG and no correlation between the levels of EE and KDG. On day 21 of intake of 30EE/DG, the serum concentrations of sex-hormone- and corticosteroid-binding globulin were higher by 16% and 12%, respectively than with 20EE/DG. Although the morning peak levels of cortisol did not differ, the decrease which occurred thereafter, according to the circadian rhythm, was slower with 30EE/DG. There was no relationship between the serum concentrations of EE and/or KDG and the occurrence of irregular bleedings, which was similar during treatment with both preparations. As most of the women who bled had bleedings both with 20EE/DG and 30EE/DG, an influence of predisposition can be assumed. It is concluded that the increase in the EE levels and partly of progestogen levels during multiple intake of the pill is probably caused by an inhibition of hepatic steroid metabolism by the contraceptive steroids.