This document discusses estrogen and progestin, female sex hormones. It defines estrogen as substances that can induce estrus in spayed animals. The three major natural estrogens are estradiol, estrone, and estriol, which vary throughout the menstrual cycle. Estradiol is the predominant estrogen during reproductive years. Synthetic estrogens like ethinyl estradiol and mestranol are orally active. Progesterone is the major natural progestin, secreted from the corpus luteum during the menstrual cycle and maintained in early pregnancy. Synthetic progestins like medroxyprogesterone acetate and desogestrel are highly active orally. Estrogens and progestins act through