Estradiol is the major estrogen secreted by the premenopausal ovary. Estrogens direct the development of the female genotype in embryogenesis and at puberty. In addition, estradiol is an important luteolytic agent in humans. Plasma estradiol levels increase gradually between days 1-7 of the menstrual cycle followed by a sharp increase to a peak value of about 300 pg/ml on day 12 just prior to ovulation. Estradiol levels in most rodent samples are extremely low, rising to detectable levels only during the pro-estrus period and in pregnant females.