Summary. Intravaginal rings containing progesterone were inserted on Day 5 of the cycle to 8 healthy, normally menstruating women. Blood samples were taken during Days 4–22 of the cycle at 2–3-day intervals.
The plasma progesterone levels obtained after the insertion were between 7·5 and 21 nmol/1. Four subjects showed no increase in plasma oestradiol concentrations. The subjects showing increased plasma oestradiol levels also showed a positive feedback on LH, resulting in ovulation or an LH peak. The results suggest that progesterone may have a local inhibitory effect on the follicular oestradiol production.