After two control cycles, 13 normally menstruating women were treated from day 5 to 25 of the third consecutive cycle with nomegestrol acetate (NOM Ac) 1.25, 2.5 or 5 mg once a day. Plasma oestradiol, FSH and LH concentrations were assayed daily from day 5 to day 25 and plasma progesterone concentrations from day 12 to day 25. Ovulation was inhibited in every woman; LH and progesterone levels were uniformly depressed. With 5 and 2.5 mg/day, plasma oestradiol remained low with high FSH values. With 1.25 mg/day, oestradiol concentrations reached levels similar to those achieved during the control follicular phase, with a concomitant decrease in FSH secretion. These results show an hypothalamic-pituitary effect and suggest an ovarian action, both responsible for a potentially useful contraceptive property.