The effect of photoperiod and position in the ovulatory sequence on plasma concentrations of luteinizing hormone during the ovulatory cycle of the hen.

The concentration of LH was studied in blood samples removed intermittently from hens. Hens were selected for study in the middle of sequences when ovulations are separated by 24 to 25 hr, before the first ovulation of a sequence when successive ovulations are separated by 40 hr, and in an 14L : 14D photoperiod when all ovulations are separated by 28 hr. A preovulatory surge of LH always preceded ovulation by 4 to 7 hr, and a nadir in the concentration of LH always occurred 10 to 11 hr before ovulation. This nadir followed a gradual decline in the concentration of LH, which began 19 hr before a midsequence ovulation, 27 to 31 hr before the first ovulation of a sequence, and 20 to 28 hr before ovulation in a 14L : 14D photoperiod. It was concluded that this decline in plasma LH terminating at a nadir 10 to 11 hr before ovulation may be involved in the acquisition of ovulability by the largest follicle in the ovarian hierarchy.