Concentrations of Circulating Gonadotropins During Various Reproductive States in Mares1

Concentrations of circulating FSH and LH were measured in pony and horse mares during the estrous cycle. The gonadotropin profiles did not differ significantly between the two types of mares. During the estrous cycle FSH and LH were secreted in a reciprocal pattern with FSH high during diestrus and LH high during estrus. Although the patterns of FSH and progesterone secretion were similar, FSH rose before progesterone at the end of estrus, indicating that the initial FSH increase was not due to a positive feedback effect of progesterone. The patterns of LH, but not of FSH secretion, diverged between nonpregnant and pregnant mares 19 days after ovulation. In both hysterectomized and pregnant mares, FSH concentrations were elevated after the end of estrus, comparable to what occurred during diestrus. Treatment of diestrous mares with estradiol or of seasonally anovulatory mares with clomiphene citrate had no significant effect on concentrations of circulating FSH or LH.