Some observations concerning the role of progesterone in the control of ovulation

Pre-ovulatory LH-surges in 4-day cyclic rats (4d-r) were postponed 1,4, or 8 days by treatment with progesterone (P), or 1 day by treatment with pentobarbitone just prior to the onset of the 'critical period' on pro-oestrus. P-delayed LH-surges tend to become higher with increasing duration of the preceding anovulatory period: 1 day-delayed surges were as high as 5d-r surges (i.e. about twice as high as spontaneous 4d-r surges); 8 days-delayed surges were significantly higher. Compared with 4d-r surges the time of onset and zenith of P-delayed surges is significantly advanced. The 1 day-pentobarbitone-delayed surges too were as high as 5d-r surges, but still timed as 4d-r surges. On no 'artificial' pro-oestrus day did the responsiveness of the pituitary differ from the 'normal' pro-oestrus responsiveness. It is concluded that P influences the timing but not the height of pre-ovulatory LH-surges.