The measurement of the urinary excretion ratio of 6 beta-hydroxycortisol (6 beta-OHC)/cortisol was used as a non-invasive method to investigate possible changes in the activity of drug-metabolizing enzymes in women receiving different oral contraceptive formulations for 1 up to 3 treatment cycles. The contraceptive preparations were either levonorgestrel, gestodene or cyproterone acetate, each in combination with ethinyl estradiol, or only the progestogens levonorgestrel or gestodene. There was either no or only a small decrease in the 6 beta-OHC/cortisol ratio. Thus, only a minor inhibitory effect, if any, can be ascribed to the investigated contraceptive steroids in vivo. Previously observed differences between selected contraceptive steroids in vitro were not observed in the same way in vivo. This may be due either to the absence of a marked inhibitory activity in vivo or to the insufficient sensitivity of the marker 6 beta-OHC/cortisol to detect these changes. Another possible reason may be the considerably higher drug concentrations used in the in vitro studies as compared to those present in the serum of women under oral contraceptive therapy.