In a cross-over study with orally administered desogestrel (0.150 mg) plus ethinyloestradiol (0.030 mg) and 3-keto-desogestrel (0.150 mg) plus ethinyloestradiol (0.030 mg) in ten women under steady-state conditions, the serum levels of 3-keto-desogestrel were monitored by radioimmunoassay. No statistically significant differences between treatment groups were found with respect to the areas under the curve of the serum levels versus time (AUC), peak heights and peak times. The individual AUCs for 3-keto-desogestrel after dosing with desogestrel (plus EE) or 3-keto-desogestrel (plus EE) show a similar degree of variation. The biotransformation of desogestrel into 3-keto-desogestrel is rapid and appears not to be limited by the metabolic capacity of the normal liver.