The concentration of 13,14-dihydro-15-oxo-prostaglandin F2 alpha (PGFM) has been measured in daily samples of peripheral venous plasma from 5 women throughout the ovarian or menstrual cycle. The values have been combined by days, relative to a pre-ovulatory peak, the LH peak and to Day 1 of the next menstrual cycle. The patterns (geometric means) have been related to the corresponding values for oestradiol, LH and progesterone. The results show that there was considerable variation in the concentration of PGFM (50-909 pmol/l; n = 140) and that the coefficient of variation within an individual cycle ranged from 30-74%. Nevertheless two peaks--one pre-ovulatory (associated with a marked increase in the level of circulating oestradiol) and one pre-menstrual (associated with rapidly falling levels of circulating oestradiol and progesterone) were apparent in all cycles. The concentration of PGFM during the pre-ovulatory rise was 521 +/- 325 pmol/l; mean +/- SD and the corresponding value for the pre-menstrual rise was 571 +/- 325; both peaks were significantly higher (P less than 0.0025, P less than 0.0005, respectively, Student's t-test) than during the remainder of the cycle (257 +/- 190). There was no significant difference, however, between the concentration during the follicular and luteal phases.