The results of the cervical mucus score (CMS), basal body temperature (BBT) and a rapid urine luteinizing hormone (LH) assay by haemagglutination immunoassay were compared with plasma LH radioimmunoassay in 28 ovulatory cycles. There was good correlation of the CMS peak with the plasma LH peak while the urine LH peak usually occurred one day later. The first day when the CMS was greater than or equal to 8 occurred within +/- 1 day of the plasma LH peak in 89.3% of cycles. The first day when the urine LH was greater than or equal to 100 iu/l occurred within +/- 1 day of the plasma LH peak in all the cycles. BBT nadir could be identified in only 82% of cycles and it coincided with the plasma LH peak in only 28.6% of cases. These findings suggest that both the CMS and the rapid urine LH assay are acceptable methods for determining the time of ovulation while the BBT is not useful.