The mean plasma testosterone level after 6 to 17 months in seven adult males receiving 100 mg oral cyproterone acetate daily, was 267 ± 125 ng/100 ml (±S.D.) with a range of 99 to 518. This represents a reduction of only 65% from the normal controls. Plasma androstenedione levels showed a mean of 125 ± 93 ng/100 ml (±S.D.) with a range of 52 to 162 which is within the normal range. The same dose of cyproterone acetate given to two female volunteers from Day 5 of the menstrual cycle for 10 days caused a reduction in urinary total oestrogen and pregnanediol to levels inconsistent with ovulation. With the addition of 0·05 mg ethinyloestradiol from Day 5 for 21 days, as in the `Reverse Sequential Regime', ovulation appeared to be inhibited in all the ten cycles studied. There were regular `withdrawal bleedings'. When the oestrogen was continued alone, ovulation returned in both women and after the cessation of all steroid treatment their normal pattern of hormone excretion continued. These results are discussed in relation to the possible mechanism of action of cyproterone acetate.