Changes in the Concentrations of Oestrone and Oestradiol in the Peripheral Plasma of the Domestic Hen During the Ovulatory Cycle

The concentrations of oestrone and oestradiol were measured by radioimmunoassay in the peripheral plasma of hens during the ovulatory cycle and of non-laying hens. During the ovulatory cycle when blood samples were taken at 4 or 6 h intervals for 24 h, the peak concentrations of oestrone and oestradiol occurred within the 6 h period immediately preceding ovulation. After the final ovulation of the clutch the oestradiol concentration fell gradually and did not rise again near the time when the last egg of the clutch was laid. There was no significant change in the concentration of oestradiol over a 24 h period in non-laying hens. When blood samples were taken at 2 h intervals during the 12–14 h period before the expected time of ovulation the oestradiol concentration rose 8 h before ovulation and reached a peak level 6–4 h beforehand. It is suggested that oestradiol may be involved in the mechanism stimulating the release of luteinizing hormone required for ovulation.