SUMMARY The excretion of urinary free ll-hydroxycorticosteroids and total oestrogens was studied daily in morning urine specimens throughout the menstrual cycle of six normal women (age range 18-24 years). The follicular phase of the cycle was characterised by apparently random fluctuations in the excretion of urinary free corticosteroids. However, after the mid-cycle oestrogen peak there occurred a significant drop in corticosteroid excretion, which then rose to a peak eight to 10 days after ovulation and was synchronous with the second oestrogen peak during the luteal phase.Results o 0 -10-9-8 -7-6-5-'.-3-2-1 0 1 2 3 L. 56 7 8910 §.Ooys relative 10 mid-cycle oestrogen peak (day 0)T he composite cycle shown in the Figure represents the mean excretion of urinary free corticosteroids and total oestrogens in six volunteer subjects. i10