Ovulation inhibition and bleeding control with a combination of 0.5 mg 17 beta-estradiol cyclo-octyl acetate (E2COA) and 0.15 mg desogestrel was investigated in 10 regularly menstruating women for 21 days. In half the group the treatment was extended for 7 days (days 22-28) with 0.03 mg desogestrel in order to evaluate any posttreatment influence on the gonadotropin levels. E2COA, beeing a long-chain fatty ester dissolved in oil, was expected to be resorbed from the intestinal wall via the lymphatic system. By incorporating it in the chylomicrons, E2COA would thereby avoid the unfavourable first liver pass. The serum levels of progesterone were suppressed during treatment. No increase in 17 beta-estradiol (E2) concentration was found; levels remained low and even during medication. No peak values of gonadotropins were seen. Thus follicular hormonal activity and ovulation was inhibited by this combination. Bleeding control was, however unacceptable in all volunteers. The addition of desogestrel during the fourth investigation week apparently did not induce any hormonal differences. The estrogenic activity is shown by the low, even S-E2 levels, but the dosage of E2COA seems to be too low in relation to progestogen dosage. Further studies will have to be performed in order to find the ideal combination.