SUMMARY: Changes in the peripheral plasma level of oestradiol during the bovine oestrous cycle were determined by a competitive protein-binding assay, following chromatographic purification of the steroid. Oestradiol concentration increased during the 3 days preceding oestrus and attained its peak value [17 ± 1·9 (s.e.m.) ng/100 ml] 4 h before oestrous behaviour could be detected. The level declined steeply during the day of oestrus to reach a nadir (0·8 ± 0·11) before the time of ovulation. A minor rise was observed on day 4 and a more sustained increase on days 10–13, with a peak on day 11 (8·1 ± 3·6). Comparison with published data on luteinizing hormone (LH) levels suggests that the pre-oestrous surge of oestradiol precedes the pre-ovulatory surge of LH in the cow.