It is known that a small fraction of orally administered norethisterone is metabolically converted to ethinylestradiol. This exploratory, open-label, nonrandomized study was conducted to investigate the systemic exposure to ethinylestradiol after intramuscular administration of norethisterone enantate in comparison with the exposure to ethinylestradiol after administration of a standard combined oral contraceptive. Sixteen healthy premenopausal women received an oral contraceptive (ethinylestradiol 30 μg/levonorgestrel 150 μg) once daily for 21 days and-after a 1-week washout period-a single intramuscular dose of 200 mg norethisterone enantate. Blood samples to determine ethinylestradiol in serum were taken over 24 hours after the last dose of ethinylestradiol/levonorgestrel and over 8 weeks after administration of norethisterone enantate. Oral equivalent doses of ethinylestradiol were estimated based on area under the concentration-time curves. The ethinylestradiol serum concentrations observed after administration of norethisterone enantate were relatively low: The mean maximum concentration was only 32% of the maximum observed after ethinylestradiol/levonorgestrel (90% confidence interval, 22.5%-44.7%). The maximum oral equivalent dose of ethinylestradiol was markedly lower than 30 μg ethinylestradiol per day (20.3 μg/day; 90% confidence interval, 14.8-28.0 μg/day). The same applied to the average oral equivalent dose of ethinylestradiol for the 8-week postdose interval (4.41 μg/day; 90% confidence interval, 3.57-5.46 μg/day). To conclude, the study results indicate that metabolic conversion of norethisterone to ethinylestradiol also occurs after intramuscular administration of 200 mg norethisterone enantate, but is associated with a lower exposure to ethinylestradiol than the use of a combined oral contraceptive containing 30 μg ethinylestradiol (plus 150 μg levonorgestrel).