The purpose of this study was to validate the dual analyte system (Boots Celltech Ltd, Slough, UK) as a marker of ovarian function. For this purpose the urinary profile of the ratio of oestrone-3-glucuronide to pregnanediol-3-glucuronide in urine (E13G/PD3G) was compared with plasma concentrations of oestradiol, progesterone, luteinizing hormone (LH) and follicle stimulating hormone from day 8-24 of the menstrual cycle in 23 women. Daily transvaginal ultrasound was also performed from day 8 of the cycle until sonographic evidence of follicular rupture. The ratio of urinary metabolites exhibited a maximum value close to ovulation. In the majority of cases this coincided with the day of LH peak, and was followed by follicular rupture within the next 24 h in 74% of cases. The assay gives a good marker of ovulation, however, it is unlikely to help in detecting the initiation of the fertile period or the quality of the menstrual cycle.