Sulfates are the key circulating conjugates of ethinyl estradiol and glucuronides predominate in urine. The principal oxidative metabolite is two-hydroxy ethinyl estradiol, formed by the CYP3A4 isoform of cytochrome P450. A part of the primary-go metabolism of ethinyl estradiol is believed to occur in gastrointestinal mucosa. Ethinyl estradiol may