1. The effect of the progestational compounds, norethisterone and norethisterone acetate, on ovarian and pituitary function during the menstrual cycle have been assessed in five subjects by measuring the urinary excretion of oestrogens, pregnanediol, pregnanetriol and pituitary gonadotrophin (HPG).
2. The effects of the two compounds were similar and depended on dosage and time of administration during the cycle. With adequate dosages given from day 5 to day 25, there was complete suppression of ovarian activity; with lower dosages given over a similar period, follicular activity could persist but ovulation and formation of a corpus luteum were suppressed. When administered during the luteal phase only, there was no apparent effect on the normal function of the corpus luteum. In none of the subjects was urinary HPG excretion depressed during drug administration.
3. The action of these compounds as ovarian or pituitary inhibitors is discussed. It is concluded that they exert their effects, not through their weakly oestrogenic activity but through a property common to synthetic progestational compounds.