Twelve cows between Days 10 and 17 post partum received one or two silicone rubber implants, each containing 45 mg oestradiol-17 beta. Plasma oestradiol concentrations increased from 4.25 +/- 0.51 pg/ml before implant insertion to 16.57 +/- 3.66 pg/ml (one implant) and 37.10 +/- 9.1 pg/ml (two implants) during the 5 days of implant treatment. Preovulatory-type LH and FSH surges were induced in 11/12 cows between 15 and 79 h after implant insertion. There were no relationships amongst plasma oestradiol concentrations, LH pulse frequency, pretreatment FSH concentrations, timing of the LH surge or time to first rise in milk progesterone concentration. There was a negative correlation (r = -0.58; P less than 0.05) between the stage post partum and the timing of the peak of the LH surge after implant insertion. Although the positive feedback mechanism was functional in most cows, the data may reflect a continuing recovery of the responsiveness of the hypothalamo-pituitary axis to the positive feedback effects of oestradiol-17 beta between Days 10 and 17 post partum.