In 6 pre-menopausal and 6 post-menopausal women we compared urine volume and excretion of creatinine and urea daily from day 2 of the 1st period of menstruation to day 1 of the 2nd, along with clearances of creatinine (CCr) and urea (C-urea). Clearances were based on 7 serum samples which were taken on day 2 of the 1st menstruation and on days 8,14,15,16, 21, and day 1 of the next menstruation, when 4 cycling hormones were also assayed. Mean group ages were 35 and 59 years for pre- and post-menopausal women. CCr cycled only in the pre-menopausal group, with a nadir around the time of ovulation. The mean CCr combining days 14 and 15 was lower than that for the average of the 5 other days by 22% (p < 0.003) in the pre-menopausal but not in the post-menopausal group (p = 0.99). Average CCr values fell from the peak mean of 129 ml/min on day 8 in the follicular phase to the mean nadir of 93 ml/min for days 14 and 15 and recovered to 117 ml/min by day 1 of the 2nd menstruation; the percent fall in CCr ranged from 9 to 39%. A mid-cycle nadir was also present for C-urea (p = 0.03), also found only in the pre-menopausal group. We conclude that, in pre-menopausal women, there is generally a fall in CCr and C-urea from a peak in the follicular phase to a nadir around ovulation.