Progesterone levels were determined in the peripheral plasma of four gilts on each day of the oestrous cycle. An initial rise was observed at Day 3 or 4 of the cycle (oestrus = Day 1) followed by a very rapid increase up to Day 7 or 8 and a slower rate of increase until Day 14 or 15 of a 20-day cycle. The highest levels of progesterone during the luteal phase were about 35 ng/ml plasma and the average concentration during Days 10 to 15 was approximately 27 ng/ml plasma. The decline in progesterone levels after Day 15 was precipitous, a thirty-fold decrease occurring in most cases within 48 hr. Progesterone levels remained low (about 0·5 ng/ml) for about 7 days during the phase of follicle growth and ovulation.
A rather consistent time interval (7 days) was observed between the decline in progesterone concentration and the onset of overt oestrus. The high levels of circulating progesterone in the gilt may suppress follicle development sufficiently during the luteal phase so that approximately 1 week is required between corpus luteum regression and ovulation.
The concentration of plasma progesterone in nine castrated boars was 0·9 ng/ml suggesting an extra-gonadal source of progesterone, possibly adrenal in origin.