Estrogen is a steroid hormone produced by the ovary in the female. Follicles on the ovary release increasing amounts of estrogen as they grow, thus increasing the amount of luteinizing hormone (LH) released by the pituitary. LH is responsible for the eventual ovulation of the egg from the ovary.

The general effects of estrogen include initiation of standing heat, dilation of the cervix, and changes in the oviducts in preparation for egg and sperm transport.

For more information, see Mare Reproduction.