When GnRH is secreted in the proper amount, the Pituitary Gland located at the base of the brain is stimulated. The Pituitary Gland then secretes 2 hormones that effect the ovaries. It travels in the bloodstream to the ovaries where it stimulates the development of one or more follicles. When the follicles reach 20 to 25 millimeters in diameter, they secrete Estrogen. LH facilitates maturation of the growing, egg bearing follicle.
Ovulation occurs when the mature egg eaves the follicle and begins it's trip through the oviduct.
Following ovulation, the Estrogen level begins to fall, and the remains of the ovulated follicle are converted to a form of Corpus Luteum, or yellow body. These Luteal cells secrete the hormone Progesterone. It is it's job to stop heat stimulating hormones, and to set the stage for maintaining pregnancy. It subdues the contracting reproductive tract, and tightens and closes the cervix. It also prohibits the secretion of FSH and LH from the Pituitary, thereby putting the mare out of heat.