Estradiol is a steroidal estrogen. There are two types, α and β, and the α type has a strong physiological effect. Wintersteiner et al. (Wintersteiner et al. 1935) have extracted from the urine of pregnant horses, and can also be obtained from the urine of pregnant women, human placenta, and pig ovaries. Also present in the testes or urine of male horses. Because it has a strong sex hormone action, it is believed that it or its ester is actually the most important sex hormone secreted by the ovaries.