A new concept for control of the estrous cycle of the ewe based on the temporal relationships between luteinizing hormone, estradiol and progesterone in peripheral serum and evidence that progesterone inhibits tonic LH secretion.

The temporal relationships between LH, estradiol and progesterone in peripheral serum of the ewe were characterized throughout the estrous cycle. Between successive preovulatory LH sruges, serum concentrations of LH fluctuated markedly in a manner indicative of pulsatile discharges. Mean serum LH and progesterone concentrations were inversely related, LH being highest during the early and late luteal phases of the estrous cycle and lowest in the mid-luteal phase. A progressive, 5-fold increase in serum LH concentrations occurred between the onset of the precipitous fall in circulating progesterone attendant to luteolysis and the initiation of the preovulatory LH surge. Two major increments in circulating estradiol were observed in each cycle, both occurring when serum LH concentrations were relatively high. One estradiol increment occurred during the early luteal phase, the other during the 2-3 days prior to onset of the preovulatory LH surge. The latter estradiol increment thus accompanied the progressive, 5-fold increase in circulating LH which precedes the LH surge. The observations are inconsistent with the view that tonic LH secretion in the ewe is solely a function of a negative feedback action of estradiol. Rather, the temporal relationships between circulating hormones, in conjunction with recent findings that progesterone can inhibit tonic LH secretion in the ewe, lead to the conclusion that progesterone plays a major role in the regulation of tonic LH secretion during the estrous cycle of sheep.