The concentrations of estrone-3-glucuronide (E1-3-G), estradiol-17 beta-glucuronide (E2-17 beta-G), estriol-16 alpha-glucuronide (E3-16 alpha-G) and LH/hCG have been measured in daily samples of early morning urine from six women who conceived during the period study. The specimens were collected from the second day of menstruation until the values for hCG were greater than 200 U/l. The aim of the study was to assess which of the metabolites should be measured preferentially to predict ovulation by 72 h and hence identify the probable start of the fertile period. The peak of LH was used as a reference point for ovulation and defined as Day 0. A defined rise (50% increase over the mean of three preceding values) in the concentration of each steroid metabolite occurred on the following days (mean and range): E1-3-G Day LH-4.7 (-3 to -6), E2-17 beta-G Day LH-3.7 (-3 to -6) and E3-16 alpha-G Day LH-4.5 (-3 to -6). The concentration (nmol/l; mean and range) of each metabolite on the day of the rise was: E1-3-G (106; 78-137), E2-17 beta-G (19; 5 to 27) and E3-16 alpha-G (28; 18-40). It is concluded that the measurement of each estrogen glucuronide provides similar information and quantitatively E1-3-G is the most important.